So he's a killer now too?? Why did he kill him? To test the police?

it was a dream

no sorry, it wasnt, my bad ^^'

I think its a dream and he killed him because he thinks that he was the one that killed his father. So Sangbea probably wanted to avenge his dad by stabbing that guy he believed to be the killer. But yeah, I don't he did it to test the police, he probably thought what else does he have to lose if he were to do it.