
Spoiler
...yup. He rapes him. Quite violently at that (it wasn't shown in detail, but in the manhwa they might considering that they added sex scenes that didn't exist in the novel). For DAYS. There was even a scene after that where a doctor visits him and tells him that a bite mark he gave him on his neck will scar and never completely heal :/ (at that point even his knights gave him a disgusted look)
The ML himself admits that he's a bastare and only did it out of jealousy instead of feeling betrayed by the fact that Radan was an assassin, and that he would have done it even if he was actually innocent...
As for Radan's reaction: this all happened while he lost his memories and DIDN'T KNOW WHAT WAS GOING ON and was terribly frightened by him. Then at some point he kind of escapes when there is a red moon/bloody moon (which, spoiler, is when Radan can open his eyes without killing anyone), he regains his memories, they find him, and we get a tearful and heartfelt reunion in which Radan becomes aware that he loves him, and they hug?!? Then Leshak is convinced he's gonna be hated due to what he has done, but Radan has no intention of doing that and never really addresses it (and Leshak wouldn't care and would confine him either way).
So yeah, he's a MASSIVE red flag

Ugh I HATE when they bait us with a sane green flag ml just to turn him into the worst scum on earth.
I'd even go as far as to say it's dishonest because they know the person reading this will want something cute (by pretending it's cute and healthy at first) and then when it becomes awful they know the reader will still read hoping it becomes better i'm dropping this
I had read the novel long before the manhwa came out and just reread it to refresh my memory, and... damn, Leshak was MUCH more of a red flag than I remembered, and he stayed like that basically until the end of the novel. You guys are NOT ready for what comes after this if they don't change it in the manhwa (⊙…⊙ )
Even his insanely loyal knights all end up agreeing that he's a crazy asshole, which I found kind of funny out of context. Even when they become a happy couple in the side stories, when him and Radan 'fight' they all without a doubt believe that it must be Leshak's fault and take Radan's side without knowing anything lmao