You can get into a relationship with someone totally normal and then find out they have mental health issues.

That's what you said, and it's pretty close to "out of nowhere." And "mental health issues" is a long way off from "violent tendencies." You understand that there are many people with mental healtg issues that would never harm someone, right? Unless the person's a psychopath, they give plenty of signs of being violent.

He had probably already mentally broken her down and abused the fuck out of her prior to this because abusers always escalate and never start off like that.

That's exactly what I'm saying. That's the opposite of what you started off saying. At some point you have to make a choice to help yourself and your child. She's not at fault for being beaten and bloodied, but no one else is responsible for her choosing to stay with a violent person.