How Do I get a guy I like to leave his girlfriend?

I know it sounds selfish but I'm confused. See this guy has cheated on his gf before but he told her and he loves her and stuff, but one day we were talking and he said he loves me and sometimes he'll say "I know this may sound bad, but I wish you came along first" then he says he's confused and he doesn't know who he wants to be with. Then one day I went to his house and I know this may sound kinda slutty...but... we made out on his couch and almost had and I stopped it and said we couldn't because he has a gf he stopped sorta after sighing and stuff and he said that he totally forgot about her(which sometimes he says while we're talking and I usually mention he has a gf when he comes up with an idea). I then asked him if he would ever cheat on me and he told me no, and then I asked if he would cheat on his gf again and he said he probably would. He said he couldn't cheat on me because it would kill him because I have gone through way too much already. Then I told him that I didn't wanna intervene with him and his gf anymore so I was going to stop talking to him. Then he was talking to one of my friends and she heard him sharpening his blade and loading his gun and she asked him what he was planning on doing and he said he wanted to kill himself because he wanted me in his life bc without me he couldn't live or what ever. So I called him later(after he cut himself on his chest) and I told him I'd talk to him. Anyways I would really appreciate some help THANKS!