I'm trying to figure out how the LI is abusive. He really isn't. Also he wasn't using him the way you're implying. They literally had a deal. It was a mutual benefiting deal, MC restores the dungeons and ML puts his friends to rest. ML even offers him a spot on the team afterwards of his own free choice.
Yes they start a physical relationship, and the ML deff does it to keep the MC stable so that they're able to continue their deal. Also you can clearly see that the ML is catching feelings though his looks so seem kinda menacing which is a weird choice. He has been nothing but straight with the MC. Nowhere abusive.
At what point is the ML abusive? I second the reply above, they both had a deal and did what's agreed upon. They both did their job, caught feelings in the way, and protected each other. It just happened that they both have their own goals in mind, and in this case MC wants to die. ML on the other hand, realized that a bit too late.
ML never abuses him in any way, he treats MC as another beneficiary to their deal. An equal. ML can be manipulative, yes. Just look at how he handled that one annoying cunt. If that sense of manipulation or maybe his smug dark-ish expression is what you call disturbing, I think you need to just reread this manhwa with a clearer mind from the start.
As for the blackmail and controlling part, I assume you're talking about the glasses cunt from before lol. He's the one you're referring to here, sweetheart.
What was their original deal? To revive Hyeon's dad? Or to clear all dunegons?
In exchange for killing the MC's friends the ML required him to 1: keep his dad "alive" so his older brother couldn't take over the guild and 2: restore the dungeons till they cleared and the ML would kill his friends for each dungeon they cleared.
spoilers
honestly I can't imagine how broken the mc felt for such a long time and much strength it took for him to stay alive. the guilt was crushing him so much. everyone is freaking out that he's gonna kill himself but tbh it doesn't sound like a bad ending if it can finally give him some peace. his life was a hell but he endured it. it must have been so lonely. he almost resolved everything he had to and can just make his own choice. staying alive while being constantly used by someone you love seems like a lose act the end. he either somehow run away to start his new life or it stays with the guy he loves who doesn't help his mental state at all and just uses him however he wants. LI is so abusive and disturbing. I can't help but think staying with him would make the mc even more miserable. I just want him to finally have freedom to not be stopped by someone blackmailing and controlling him. if suicide is what makes his final moments his own life then it sounds like a bittersweet ending that was meant to happen. ofc he won't die but someone how I don't want it to be "and they all lived happily ever after" ending. it feels so wrong and disturbing.