I called my mother to tell her to drop dead
Why would you tell her to do that?
She was so cruel to my father who died a while ago.
It is rude to tell someone to die. You should apologize and try to save the relationship if you can.
I should apologize when my mother is a real nasty piece of work?
Yes, you should apologize to your mother for telling her you wish she was dead. This was a mean-spirited attack on your mother that no one should have to experience. Forgive your mom and show her kindness instead of animosity.