For you to end up on death row for murdering your dad would definitely not help your mom or do you any good. I do understand the impulse. It’s hard to watch someone you love be humiliated day after day. I wonder if your mom has become so used to it that she doesn’t understand that being mimicked and put down is a kind of psychological abuse. It’s also hard to watch one parent hurt the other. I wonder if your dad is so used to handling conflict this way that he doesn’t recognize that it isn’t acceptable behavior.
I don’t know enough about the situation to know if divorce would be easier. Something is keeping things miserable but stable. If the “something” is your mother’s fear of leaving, then please give her this phone number for the National Domestic Violence Hotline: (800) 799-7233. Counselors there provide support and help 24/7.
If instead your parents can’t live with each other and can’t live without each other either, you may be able to tell them that they are making themselves and you miserable and to suggest they get some couples counseling. You might even do a little research and find out the names of good therapists in your community so you could hand them the phone numbers.
At 16, you may be mature enough to handle this. If not, please consider talking to a trusted relative or your school counselor about the situation and asking for some help. The current situation isn’t healthy for any of you.
I wish you well.