why did jay do that i don’t understand

I think Jay slit his wrist (attempted suicide) in front of Yoohan because he wanted him to understand how he felt when he saw Yoohan wanting to commit suicide. Hope that makes sense

I think its also a way of Jay saying that if Yoohan kills himself he'll be killing Jay as well

Same. What the? What is this some kind of tantrum? He behaves like a pubescent 15yo.

Its to make yoohan feel guilty because guilt was the only reason he hasn’t killed himself yet

It's two traumatized people who have to go to therapy. I was also surprised to read it the first time, but Jay has a low tolerance for frustration and he has volatile reactions to things (beating people, and destroying their lives because they don't do what he wants) so this makes sort of sense.
It doesn't make it right, but it kind of makes sense. Especially since Jay saw the suicide scene (too realistic), and Taemin wanted to die before, so Jay did the asshole move of: if I kill myself and you feel guilty you would live right? Because Taemin lived because of the guilt for his family (repaying his family)