What is wrong with my mother?

[phoenix]

My father is the kindest man I have ever known. He will let people walk all over him if it means he can make them happy. He is a devout Christian and he lives by the bible, and has never hated a single soul. He tried to raise me and teach me love and compassion for others, which I greatly appreciate.
My mother is the problem.
Her childhood was hard. Her father had Schizophrenia and her mother was always working because her father could not. She has had counseling to help her deal with the problems that she had to face because of her childhood... but it hasn't helped. She needs something more.
Ever since I was born, my mother has been extremely selfish and has taken advantage of my father and his family. She didn't know how to properly breastfeed me and I almost starved to death because she couldn't be bothered to learn. Growing up, I remember her being very paranoid about people around her. She was very isolated, didn't have friends, and would always be insecure. Her insecurity drove her hatred in other people... including me.
I'd never been a problematic child. I was smart, made friends, never got into trouble, but I was a sensitive child and would cry if someone said something hurtful to me. She would constantly put me down, make fun of me and laugh at me despite this. She would often tell me she wished I had never been born, that I was a burden, etc, and she hit me a few times which really rattled me. She didn't have a job and wasn't trying to find one, and she wasn't cleaning the house either. Our home was always a mess and my dad had to clean most of the time. All she would do all day is lie around or call her mother.
As I grew older, it only got worse. Her insecurities turned into an obsession; she constantly asked my dad for his opinion on her looks, many many times a day. He was obviously annoyed but went along with it. Her paranoia reached to every corner of her life and she constantly asked me what drugs I was doing or who I was hanging out with (Like I said, I was a good kid, and my father backed me up on that.) Every time she and I were alone, she would scream and swear at me (which was a big deal within our Christian family) and would tell me how much she hated me. She would neglect my sister and I, hardly ever buying food or clothes for us, wouldn't drive us anywhere or let us do anything to the point where we felt trapped in our own home.
Then... the final straw for me.
I discovered that my mother was cheating on my father through some messages that I wasn't supposed to see open on my dad's computer when I was borrowing it. I panicked and called my aunt, who I can trust with these issues, and she told me to talk to my dad about it, but I didn't want him to know that I had seen his private messages. He was swearing at my mother, telling her to "get the f*ck out" after all he had done for her.. and she was begging him to call her, begging over and over. He gave in. He let her manipulate him again. She had cheated on my father, after all she put him through, and he forgave her.
I'm furious with her. I want them to split. I hate what my mother has done. My philosophy in life is to respect others, but I don't respect my mother after the neglect and pain that she has put me through.
I have told people I trust about her issues and they've all said she's crazy. I think she might have something similar to her father, but I have no idea. She has only made my mental illnesses worse with her own delusions, and It's not healthy for me to be assosiated with her anymore. I don't know what to do to help my father at this point. I'm just stuck.