Huh?

It's because the translation isn't correct.

Hmm... not sure fixing the translation will change the fact that the demon king is the nicer and more likable character.

I can re-translate it. Hold on. I have the original Japanese volume.

Perhaps reread chapter 1? It explains it all there

This is my rough translation starting from the second page—I will skip most SFXs since they’re not important. Note that I only do this because I feel bad for people who don’t understand Japanese and can’t understand what’s going on. My translation isn’t for scanlation, if you want to use it you need my permission:
Why are you running away? | Even though they were turned into underlings for the sake of your growing army.
Mado!! That’s enough! It’s impossible!!
floating text: (He came) from behind!!

Why, you simply grace these dregs with your kind greetings, right?
Which is almost the same as forced enlistment!! [Note: He is referring to the concept of the growing army.]

Since I cooperated (with you) for a bit, isn’t it fine already!! | I’m leaving!!
Mao-san.

*sighs*

Even though I just wanted to hang out with my friends on my day off…
small floating text: It (always) ends up with me converting the other parties into my slaves right away.

…In the end it’s just me. (Guess I’ll just kill) time by playing some game…hm?

p3
…Why is there a dust plug on my phone?
floating text: I don’t remember buying one…

…Wait a min, could it be… | Nah, still, it can’t be.
floating text: Would he go so far to do something like-

Were you waiting?
…I knew it | small text: I wasn’t waiting…

For us to run into each other downtown by pure chance, you should be happy.
Bollocks to that ‘pure chance’…!!

p4
There’s probably something about this dust plug…
A hero must always keep taps on the demon king’s whereabouts.
What about privacy!

…Oh well, whatever. | Are you free…?
small floating text: So you’re fine with it?

…
Are you inviting me? / Are you seducing me?

…Well…
…What should I say, right, since you (always) take care of most of the expenses, | I’m thinking that I should at least treat you to a coffee…

p5
Heh…
I see… Thank you.

It- | It’s nothing.
It’s kinda weird seeing him docile.

Getting all happy over a cup of coffee…
…You’re fine with me choosing a place, right…

No, I’ll be the one deciding where. | Kaname, prepare the car.
Yes, sir.
floating text: You were here, Kaname-san!?

What I’m gonna do if it’s over a thousand yen per cup…

p6
Isn’t this home?!
I feel bad letting a guy with no money treat me.
Thank you for being considerate!!

And there I was trying to give and take…Well it’s true I have no money though…

Give and take? That’s not necessary.

…
Ah… | I see.

p7
What the, didn’t you just happily thank me earlier…Whatever.

…Then what about all the [i]I like you, I love you[/i] up until now… | Were they just jokes?

Oh well, it wasn’t like I was serious about it?!
small floating text: hm?

Hey, Mao. | What are you depressed about?

…

Nothing? Didn’t you just say you don’t need give and take? Then don’t come near me. (Note: He means that even his allowing him to approach is part of the give and take concept)
…Well, why would I need that,

p8
when I can just press you down by sheer force. Don’t say something soft.
floating text near Kamigo: *sighs*
floating text near Mao: You’re really dangerous…
I knew it…

Still, how can he shamelessly say it every single time… (rhetorical)
floating text: what?

…You know, Kamigo, | even I properly learnt it…

The way you talk is unclear sometimes… [Note: He speaks with too few words.] | You should speak in a way people can understand better.
Deal with it.

Stop being unreasonable!! | Seriously, imagine my feeling of being twisted around somebody’s little finger (all the time).
(Do you feel) like you are being twisted around by me?

*smiles smiles*
Heh…
*annoys*

p9
…You seriously get on my nerves. | We haven’t done it in a while, let’s have a match!!
small floating text next to the bubble: Alcohol’s fine, right!!
floating text below: I’m in good shape of late!!

What d’you mean “in a while”? Then is it alright for me to consider what we’ve been doing up until as normal sex and not us having matches?
No?! I don’t mean it like that, I’m talking about a serious match?!
Whatever, except for that one time with the eyeglass dude, you’ve always been utterly defeated.
…!!

Where should I press to bring the you from ‘that time’? …Inside your mouth?
How would I know!! To begin with, you always pressed that spot, didn’t you?

…

…So I’ve repeatedly hit there, huh?
…Yeah.

…If I really think about it, be it kissing or sex, he’s always been my partner. It’s too late to complain about it now… | Thanks to that, only my XP in that area has gone up...
SFX: *nudging closer*

p10
Alright…

I will (gladly) enter a match no matter how many times. Even if it’s [small text]sex[/small text]!!
Shall we start now?!
Nope.

*grabs*
Eh?

Now is a coffee time. Don’t be so excited. Calm down.
floating text next to his head: *sighs*
…What’s this. Somehow I feel unsatisfied…!!
floating text: Where did the usual overbearing attitude go?

So actually the hero is quite cute too. >.> That translation of chapter 10.5 is just totally off the mark.

Wow! Thank you for taking the time to do all that.
Do you do translations often?
I imagine a lot of groups would love to have someone like you helping out if you were available...
-
That being said, I still can't see the hero as a good guy. For me, he comes across as arrogant, which I find incredibly unattractive. While the demon king seems both humble and genuinely caring. It's almost as though the author reversed their personalities, but I don't get why.
I imagine it would be funnier if the demon king was the boastful jerk, while the hero was a sweet and modest guy who kept getting the better of him.
I'm definitely going to keep reading this, so my opinion may change, but I'm still rooting for the demon king and thinking he could find a better partner than the hero.

Ah, I have my own scans group. :> I'm the founder of Whatever scanlation--dunno if you've read anything of mine, if you read BL then maybe Red Theatre, Zhen, if shoujo then maybe Vampire Library, if seinen then maybe Ijinkan or Kasane, these are our current projects. It wasn't so much work, all I did was writing a quick translation. I felt it was a pity that people didn't get the right information. lol I only did it on impulse.

He isn't that awful really. It just didn't come out well in the current scans' translation. (no offence meant to the scanlator in anyway. I know ppl appreciate that it's even scanlated at all). The hero is just very blunt rather than being overly arrogant. hahaha. Vol 3 is very cute. You will love him a lot in vol 3, I know I do. He tries so hard just to get his love reciprocated.

I have read Vampire Diary and will look into the others.
Many thanks for all the time you put into making stories available to readers like me because I really love Japanese mangas, but commercial translation of more obscure ones only ever seem to come from fans.
I will also try to give the hero the benefit of the doubt...

Well uh, I didn't mean to advertise. lol But thank you. You don't have to force yourself to read my ongoing projects though. They are not very healthy and honestly they are consisted of various genre. Not just BL.
Although I wonder if I shouldn't have re-translated it, might have hurt somebody's feeling. lol... Sb else also asked so I just quickly did it arbitrarily out of goodwill...