I think the only person that saw it as friendly rivalry was Jinhyun. Sangmin gave him food poisoning the day of their exams in school remember? At that time, it's easy to let it go and say they were just kids. But now he's an adult and he's literally a serial murderer. He's an adult. There are other ways to sabotage instead of killing. Yes, his father is an insane and negative influence on him but you can't pass the buck like that. Everything Sangmin is doing, is of his own volition. No one is telling him to kill. He just thinks that's the best way to get his way. Life is made up of choices. And this is the way he's choosing to live. If he's so tired and haggard, he can stop. He can go to another hospital. Live his life away from his father's influence. Work with Jinhyun to take his father down. But he's not making those decisions. He's a weak boy with a terrible attitude and lack of morality.
And, you are right. Somethiing is going to happen to him soon. So far he hasn't been held accountable for any of his wrong doings. There will be a moment where he will hit rock bottom and his whole glass house will fall apart. And that will be moment he changes.
Sorry, I didn't mean to write a whole novel. But my sympathies for this fictional person extend only so far.
(not really spoilers but more of a rant??) when i first started reading this, i got so hooked and i still am. but, the fact that lee sangmin is repeatedly being used for some drama in the story is starting to hurt my brain. it's true that he was the cause of a lot of the bad crap that's been happening to the mc, BUT IF YOU TAKE THE TIME TO THINK, the chairman is really the one at fault because he's been constantly harassing lee sangmin, on top of comparing him to the mc on multiple occasions. even in the latest chapters, look how haggard lee sangmin has become. i honestly think something is going to happen to him soon bc of all the crap that's been happening. i missed when he and the mc had a friendly rivalry, but i guess it can't be helped. and what a plot twist that he's dating the mc's ex-wife now 0_____o... i'm not saying that lee sangmin is a nice person by any means, but i wish the author could give him a break (and also a break for the mc too please -___-)...