I wish that my mother-in-law had been in her car when it caught fire last weekend.
Why would you wish harm on your mother-in-law?
Because she's so mean.
While its not uncommon to not get along with your in-laws, its wrong to wish they were killed or injured in an accident. Whether you like it or not they're a part of your family now and you should do your best to get along with her for your family's sake.
She's a bitch. I'm going to do my best to make sure that my husband is miserable for the rest of his life.
It sounds like your mother-in-law it's very difficult for you to deal with and your relationship with her is negatively affecting your life. If your mother-in-law isn't treating you well, then you need to figure out how to remove her from your life. How you do that should be between you, your husband, and a mental health professional.