What do i say without getting her mad?

Well, you should be honest with her and tell her how you feel and also what you've been doing and when you argue don't just ignore the problem and ignore her, you need to talk and build communication skills, be honest with each other. I'm sure you've heard it all before but you shouldn't cut or hurt yourself its very bad for you physical and emotional health. To me your friend sounds like she needs therapy, if her automatic response to someone getting upset is killing herself it sounds like a defense mechanism. I don't know how old you both are so I can't give advice and I don't know the exact situation but you guys need to have a long conversation about this

I know i shouldntcut but its just a way of letting go of the pain i feel inside, if that makes any sense, i talked to her and i told her to tell someone like her mom, she said she would, so thats good :) if she dosnt tell her mom i think i might tell someone, i know shes going to get really upset but its better to tell someone before it gets to serious... right, should i?

I understand why you cut and I don't judge you for it, and yes you should defiantly tell your mom or her parents if she doesn't on her own, she has to understand what she's saying when she says she's going to kill herself and how serious it is, but make sure to be honest with each other. Sometime you have to hurt people you care about to help them, if she gets mad make sure she understands you told them because you care and don't want her to hurt herself but you also need to think about yourself as well :)

Okay thank you so much <3