I'm going to tell my ex I wouldn't care if she killed herself
Why do you want to tell your ex something hurtful?
I probably broke my ex-girlfriend trust with me by not replying to her texts, because I was dating someone else.
You should never encourage someone to harm themself in anyway after you've hurt them. Instead, you should sincerly apologize for the mistakes you made that destroyed your relationship. Owning your mistakes will help you to take wiser actions in the future.
So what? I'll tell her she's a piece of trash anyways.
How can you be so hurtful to someone else, especially to someone you loved before? How is she going to feel hearing all these things from you, whom she once trusted? How about just leaving her alone since she is an ex, so you can move on and work on bettering yourself?