Get the police involved. Settle shit, yourself and turn em into a bloody pulp.
Countering "kill yourself" with "kill yourself" just brings you down to there level

if someone told your best friend to kill herself, what would you do?

Oi. Just because they are a hot dumpster fire of a person, doesn't mean you should tell em to kill them selves. I don't support them, not one bit. But telling someone to kill themselves is a completely shitty thing to do. You can be mad from hell and back but telling someone to off emself is still wrong.