Issues with an agressive father

I really didn't know what is going on with my father. He has been a loving father to me as far as I remember. Although he has a past of cheating on my mother, dealing with his business (he never had time for me and my mom raised me on her own basically).
I am now 23 years old, live a stable life, got my college degree and moved out recently. This issue is still bugging me somehow.

My parents have been together for 29 years and my father was always abusive in the relationship. It goes like this, at first, he is loving towards my mother, he seemingly cares for her. All of a sudden, he starts accusing her of cheating on him, even with multiple men. His accusations are full of detail and it is absolutely gross. He is/was even physically abusive. How loving he may be as a father, he seems to ignore my presence when humilating my mother. He verbally destroys her. He says the most disgusting things, I have never heard anybody speak in that manner. It seems like he wants to humilate her to the bone. He gets phisically abusive in front of me. He has beaten my mother way too many times.

The weirdest thing about all this is that his acccusations never make sense. My mother has never cheated on him. Sometimes, he accuses her of certain things, and I can even witness that she really did not do it. He then verbally attacks me for protecting her and claiming that I am lying as well. My mother is the most humble and honest person I ever met. He keeps accusing her of cheating on him, of having an extra phone card, of using drugs to organize orgies. He once even claimed that my mother sneaks out late at night when he is not at home, not knowing that she's been up all night to take care of my while I had my first period cramps.

He has never been home, he cheated on my mother multiple times (we have proof, although he keeps claiming that we are lying). He constantly keeps threathening my mother, saying things like: 'I will beat you so hard, you will end up in a wheelchair.' My mother has always been a lot more beautiful then he is. He pulls out her hair, he hits her directly in the face so it swells (my assumption: to make her 'ugly'). He regularily spits at her, he keeps her up all night, he threatened to kill her, even holding a knife under her neck (I saved her from him on that point).

At some point, he gives her compliments, buys her flowers etc. The next moment he tells her she should not 'relax too much only because he is nice to her'. He constantly claims that he has 'proof' for her behaviour. When asking him about it, he finds an excuse for not showing it.

He is 100% convinced, no matter how much proof I present him that my mother is actually innocent, that she is lying. When I was younger, I asked him to go to therapy. He got aggressive, claiming that everything will be allright when my mother stops 'cheating'. I cannot describe how disgusting his descriptions of her alleged behavior are (really disgusting sexually explicit things like incest, diseases, drug-fueled orgies, prostitutions etc.).

My mom wanted to leave him, but he threatened to kill us and to 'cause us many problems' if she would consider that options. He is loving in his calm moments, but he is always ready to destroy her, even though some of his actions destroyed me or seriously threatened my life. It seems like he does not care for the effects of his behaviour on me. My life was threatened as well in many cases, and he kept going on. He has zero considerations and, the most scary thing, he ACTUALLY BELIEVES IN HIS OWN LIES AND GETS AGGRESSIVE BASED ON LIES. He attacks you if you proof him he is lying.

About my dad:
*He has always been agressive (my aunt and my grandmother confirmed this).
*He lost his father early.
*His sisters were promiscuous
*He has cheated on my mom.
*Even in his youth, he was secretive about his dating behaviours.
*He used to use women for sex and treat them like sh*t afterwards.
*He is extremely protective of me and possesive of my mother.
*He is extremely into PDA.
*He brags with me and my mother in public (no-one of our friends would expect that he is actually like I described him, everyone thinks he is a super-nice, warm-hearted familyman)
*He is manipulative
*He DOES feel sad for children who experienced, but when I ask him if he is sorry for what he has done to my mother and me he says he had to do it to 'save the family'.
*He feels sad for a woman that has rheumatic pain. My mother has the same issue and he says she has got it from 'sleeping around'.
*After he has beaten her so much that she has pain and loses all appetite, he begs her to eat after a couple of hours after his rant passes, displaying some weird kind of pity.
*He calls her 'dirty'.

I want to help him and I feel sorry for my mother? Did any professionals experience a case like this? What could be wrong with this man? What kind of mental illness does he have.

I am sorry for making this topic too long and for giving you some gross details. I did that, because I believe the way he talks could possibly say much about his personal mental illness (I do not doubt that he has a certain illness, the only question is which one).

Thank you in advance.