I think

But he surely seemed happy in S1 when we saw him killing random ppl or fucking with Bum's mind??

He's manic depressant with split personalities and severe psychological damage from chronic childhood abuse and ptsd. He acts as his father when killing to disassociate from the act and the nightmares and hallucinations he has with his mother are his true feelings and conscience constantly tormenting him for becoming like the person he hated most in the world.

I know.
That doesn't matter though.

Then you'd prolly know he wasn't happy, his mind is broken and he copes with it by dissociating himself from reality to vent his agonizing mental anguish through violence because he was never taught to healthily deal with his problems since the people who were supposed to be teaching him how to do that were too busy beating him and murdering eachother.

Nice.
Regular Serial Killer and Abuser past.
And?

Also, you can't tell me he wasn't happy when he fucked with Bum's and the older man's mind or hanged Bum.
He was smiling in every one of these, but sure feel sorry for him and don't give a fuck about his victims.

I don't think a smile demonstrate that he was happy. He was pretending to be happy, to be enjoying it, because, in a further introspection of his personality disorder and ptsd, he is just tormenting himself, too.

I guess?
I'm pretty sure every horrible human being is hurt on the inside and pretend to be the strong one.
This doesn't make it right though and didn't stop them to be thrown in jail, whether mental ilness&hurting on the inside or not, the government doesn't give a fuck about that, what counts if they killed, abused, tortured or raped and how many.

*illness.

Well, I'm not saying Sangwoo shouldn't go to jail because all the things he has suffered. He has hurt and killed many people and he has to be punished because of that.

What I was saying at first is that he is already having the worst of times before dying and that is the most painful death he is having just because he is living.