Look I get it. More than you might know.
But you realise that because you grew up in an unhealthy situation you're normal and that's probably why your mother is staying because this was and is her normal.
The problem with saying "it's who he is" is it codifying his actions into normal or in the very least a range of normal. It is not normal behaviour because not all men are like this and most would be horrifying even behaving half as badly. But because he's your dad, it's easier to be pissed at your mum than hold him accountable because he is an abuser.
What do you think about Ted bundy. Would you say "ah well he is who he is , it was those silly vitcims fault"? If the answer is no (and I hope to god it is)
I bet your thinking it's different dad isn't a serial killer... that should signal to you that means your normal is so warped that you think on some level his actions are somewhat justified and aren't probably the best mental state to comment. You need to get to therapy asap because even if your mum and dad split or he died you have grown up in a family where lots of "your normal" so you won't be able to quickly identify when your in similar situations opposed to others who didn't grow up in this environment.
I say all of the above and share one last thing, it's shit that you have to deal with this, with all the painful work therapy will throw up (and if you think you don't need therapy then that should be at least another red flag to signal you do)
You're wavering because you posted here something has gotten outside even your parameter of normal which means something bad . I wavered like you and came on here and got good advice. You won't want to hear any of this but my god do you need to hear it.
It takes victims of abuse 8 times to attempt to leave before they do (if they survive leaving)
Also your distain for your mum is probably a reflection of what your dad's shown your mum over the years. She deserves compassion, not distain but I do understand.