I'm going to make my mom depressed after giving her an ultimatum of choosing me or my step dad
What was the ultimatum?
I told her that if she wants to stay with my step dad, it's fine with me. I'm leaving. I can't tolerate the abuse from him anymore.
If your stepdad is a problem it's good to put distance between yourself and him. But don't punish your mom, she can't help who she loves.
She should choose me, because I am her child. I can't understand why a mother wouldn't.
Your mother is probably doing the best she can under the circumstances. Being away from them for awhile might make her realize you should come first. But if she is prone to getting depressed then she would not be happy either way.