We arent reading the same story istg. Use your thinking cap bro. He has a split personality, one developed by repeated trauma as a child.
He is NOT a psycho btw, nor does he want to kill more people, what even is that assumption?
When we see him choke manager, that’s the alter personality (wicked shinwoo) waking up and basically door matting his real personality. It’s clearly stated too, that Shinwoo wasn’t like that before manager found him, literally said he was grateful and all that.

Bro got trauma by killing people so he became a psycho who don't hesitate to kill more? Ew