You really stopped seeing N for any reasons. The unwritten rule is if your guys really break up, you should not see  or talk to each other for at least three months. If you kept talking to N or letting him see you, he won't take the break-up seriously. You are sending out mixed messages. By the way, I think usually Americans do not threaten things like " I will kill him or kill you"--at least educated Americans. When N says that, you should take it seriously. You should stay away from him. Normal people do not say that.

It is a really messy situation. First, kick N out of your life. Second, forget about W. He is not a really good guy either. LZmm, maybe you should try dating a Chinese guy. If the Chinese guy is wsn, at least you could quickly tell instead of wondering it is because of cultural difference.