Because it was safer. The ML kind of went crazy after the MC died. He couldn't guarantee that he wouldn't kill him, so he decided to send him to "the safest place."
So, originally, the MC and ML met as children before they went to Kunlun. The MC was an orphaned beggar, and the ML was the bastard son of a noble family. The ML went to Kunlun to survive, and to keep his people from being killed--anyone close to him were killed by his stepmother to protect her daughter's position, so the only way to protect them was to leave. But the MC didn't have anywhere else to go, so he followed the ML to Kunlun, and also apprenticed there. Unfortunately, an orphaned beggar has no value, so his teacher never taught him anything--he gave him martial arts texts to study (the MC was illiterate) and said that he wouldn't teach him until the MC learned everything in the text.
Of course, the ML had no idea that the MC was being discriminated against, because they had different teachers (the ML's teacher died during the attack) and the ML was trying his best to treat the MC like every other disciple, because he knew that he was still being spied on and he wasn't strong enough to protect anyone yet. So he wasn't there, and because everyone was hiding it from him, he didn't know.
Now, as for why Kunlun was attacked 20 years ago, they weren't the only target. The Demonic Faction attacked several places at once, and abducted more than one hundred people (including the ML). This is because the Heavenly Demon had an advisor who was basically a mad scientist, who wanted subjects for his human experiments. We aren't told what all of the experiments are, but they were creating zombies, they were increasing a person's regenerative abily, and they were forcing martial artists to learn magic (note that ki and magic are incompatible and in virtually all cases, end in death).
The ML was naturally part of the third group, because he was the rising star of the Good Faction, and the Demonic Faction really wanted to corrupt him. And it worked--the ML was the only survivor, out of ALL of them; all of the other experimental subjects died. He clung to life desperately, no matter what happened, because the MC told him to "live." Even though he was in so much despair that he wanted to give up, he couldn't, and that made him hate the MC a bit.
The Heavenly Demon used the ML as an assassin, and he did his best to circumvent him while still following his orders to the letter--warning Clans before he came to attack, etcetera. During one of these missions, one of the Neutral Faction masters tells him what was done with the MC's body (he was a nobody, so his corpse was cremated and his ashes were scattered by the river where the ML later found the MC in chapter 1), and not long after that, the ML starts making anonymous donations to Kunlun out of guilt, because they were barely scraping by after the Demonic Faction attacked--they were flat broke and at risk of starvation.
A bit after that, the ML underwent an awakening (the ageless body kind), and he killed the Heavenly Demon, and everyone involved with the human experiments, and every member of the Demonic Faction who resisted. The Demonic Faction naturally elected him as the new Heavenly Demon, but he still leaves their mountains often to indulge in his merchant/information broker/assassination guild(s), and visit the MC's "grave" every year.
That's all the ML's backstory I know. What else would you like to hear? ٩(๑❛ᴗ❛๑)۶
Hmm...next chapter, the ML gives the MC his business. (MC can still barely read, so Wi Jinyoung gives him lessons for a bit, but the novel mostly skips that part.) And the ML assigns Samrang to him as a bodyguard.
Then there's two business arcs--first is the MC dealing with embezzlment, and then second MC meets the Namgung clan heir (who also happens to be a guide) during a business trip, and gets kidnapped by pirates. But the Pirate King is actually one of the ML's followers, so he's never in danger, but outsiders don't know that. ╮( ̄▽ ̄)╭
Those two arcs aren't really all that interesting, and kind of forgettable, to be honest.
And apparently I wasn't paying much attention to the next arc, either, because I don't remember the REASON (I think they wanted to smuggle goods and needed an excuse for a lot of guards?), but the MC disguises himself as a bride and has a full wedding procession that gets attacked, and after the guards fight everybody off they discover that the MC was poison, and Samrang takes him to the ML's information guild for treatment, which is where they find out it was a multilayer poison, and after the life-threatening half is neutralized it actives a highly potent aphrodisiac that will drive a person insane if it's not dealt with.
So the ML helps him deal with it. The ML is disguised, by the way (he has a dedicated persona for operating the information guild), but when MC wakes up, he recognizes him instantly--but pretends not to know because the only reason he knows is because he recognizes the ML's guiding.
After that, they capture the traitor who poisoned the MC, and return him to the Pirate King (the traitor's ex-husband, who sold him out for profit--the traitor btw was why MC got kidnapped earlier; the Pirate King just wants his wife back so he can get revenge).
The next arc is kinda lackluster, since the only important things are, the MC makes friends with the Namgung heir, and decides to play mind games with the ML, which leads to him starting a friends-with-benefits situation with the ML-in-disguise that goes on for MONTHS. Have to get that guiding in there somehow. (≧∀≦)
My mind gets a bit fuzzy about details around here, so I'm going to go refresh my memory. But that's spoilers for the first 100+ chapters, at least.
Okay, I refreshed my memory. Back to spoilers.
MC meets the ML's niece while out shopping for clothes, and loses his mind with jealousy, because she looks exactly like ML. And he goes straight to the information guild and demands that disguised-ML investigate the ML, to see how they're related. (He suspects the ML was sleeping around after MC died and just has mountains of babies out there.) The ML is stunned and disbelieving, because he's only ever slept with the MC, but he's also filled with self-loathing and as disguised-ML, he starts talking trash about himself, which leads to them having a fight and breaking up. And disguised-ML sends MC several gifts as bribes, trying to get his forgiveness.
After that, the ML's birthday is coming up, and the MC buys him a present, and decides to deliver it to him while the ML is asleep--not knowing that the ML sleepwalks and will try to kill whoever enters the room while he's sleeping. The ML tries to strangle the MC, saying that the MC never should have saved him, etcetera. The MC gets away, and because it was his guide who attacked him (and the ML is constantly leaking guiding energy), there's no sign of injury. But the ML is suspicious that he slept so easily, and finds clues that the MC DID come into his room while he slept.
They have a surprise birthday party for the ML, and the MC interrogates Wi Jinyoung (with ML's permission) to find out what happened to ML after he died. And the ML's paranoia leads to him planning to abduct the MC and lock up up on the Heavenly Demon's base up in the mountains, but MC defuses that plan before it can really get going by making up with disguised-ML, and they start sleeping together again.
After that, there's another business arc, and MC and disguised-ML break up AGAIN, because disguised-ML asks MC to leave ML and just be with him, and asks if ML kicks MC out, will he have a chance with MC? (I swear, this is the only novel where the 1st ML and 2nd ML are the SAME PERSON wearing different hats.) (≧∀≦)
Meanwhile, the Namgung heir investigates MC, because he's curious about his benefactor (and jealous of MC and ML's relationship). And he's very prone to jumping to conclusions, because he finds out about dead-MC, and thinks that ML just abducted someone who looks like him (MC), and is abusing him (because of how damaged MC's body was when he returned). It's set up like a comedy of errors, but leads to Namgung and Kunlun teaming up against ML to "rescue" MC. So it's like, 3rd ML-turned-antagonist(?), which still hasn't been fully resolved by where I'm at in the novel (chapter 250).
Back to where I left off (150-160), MC and ML are traveling and their carriage gets attacked by zombies (hinting at upcoming arcs). After that, MC starts picking out new fabric samples for clothes for ML (because he's a simp), which leads to them FINALLY getting together officially. (づ ̄ ³ ̄)づ
So, I have to say, this novel has SO MANY bedroom scenes. When MC was with disguised-ML, it was something like every 20 chapters, there'd be another one. When MC and ML get together officially, it's EVERY SINGLE DAY, so every 5-10 chapters, there's another one. They are all over each other. But they still keep so many secrets!
ML and MC go on a date, and spend the day shopping. They split up (ML guarding the luggage while MC buys something for them to eat), and the Namgung heir abducts MC and runs away with him. He reveals that ML is the Heavenly Demon, and says he's kidnapped MC as a replacement for dead-MC, which is when ML catches up and takes MC back. And ML's paranoia is at an all-time high, so he drugs MC and smuggles him to the Heavenly Demon's base.
After MC wakes up, he gets Samrang to tell him what happened to ML for him to become the Heavely Demon, and then he blows off some steam by completely destroying every building that ever imprisoned ML. And ML is just happy, because he takes it as proof that he's loved. (▰˘◡˘▰)
ML schedules renovations on the Heavenly Demon's pavilion, and warns MC to stay out because there are magic barriers inside and the ongoing renovations mean that EVERY room is dangerous now. But MC completely spaces it, and he gets caught up in an illusion of the terrorist attack from chapter 1. MC mistakes ML for one of the terrorist espers, and fights off "them" in a desperate attempt to protect his parents (the bed), and almost goes berserk before he notices that the "esper" he's fighting is a guide.
When MC wakes up, he makes a slip of the tongue and accidentally tells the ML that this is his second life. And he tells ML that he's an esper with electricity powers, which is why he can't learn martial arts anymore. ML is obviously STOKED, because a, it really IS his MC, and also, MC is the first person who's ever pushed him so hard in a fight, even if he is nothing like a martial artist. They're both aware that people will mistake MC's esper powers for magic, so ML decides to have MC pretend he revived a dead martial art and is it's only practioner, and ML learns that martial art, too, just so he can teach it to MC. So our strong MC gets even STRONGER.
After that, the Demonic Faction pressure ML to host a martial arts tournament in the ML's honor, which is basically just an excuse for half of the surviving heads of the faction to try to assassinate the ML, with zombie suicide bombers. The attempt fails (ML is too strong), but the heads escape. For now...
Which gets us to where I am, with MC escaping from the Heavenly Demon mountains and trying to restore the ML's honor, and maybe get revenge on Kunlun while he's at it. ლ(´ڡ`ლ)
This is one of those rare novels where the guide is the top, so the ML.
There's even a bit where the MC is surprised that the ML has so much more stamina than he does, too, because they ALWAYS keep going until MC passes out...and the ML still keeps going. It is ridiculous how thirsty this man is. ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄
Again, ML went a little (read: a LOT) crazy after MC died. Also, the English translation hasn't gotten as far in the novel as where I am MTLing, so I don't know exactly if it's a case of unreliable narrator or just lack of proper nouns that complicates things. ╮( ̄▽ ̄)╭
But while ML is aware that MC died that day, he also "never saw his corpse." Coincidentally, NOBODY saw MC's corpse except for his trash master, who burned it with the rest of the garbage and didn't even separate his bones...!! If the Beggar master didn't actually care about MC, ML never would have learned where MC's ashes were scattered. ╥﹏╥
So ML never doubts that MC is past-MC. He's actually insane, but he was locked up in a dungeon full of people who were getting experimented on for over a decade. He sees MC's crazy regeneration and thinks that MC was experimented on, too. Even after MC tells him that he died and came back, ML still thinks that MC was experimented on.

I really need spoilers