Is it still abuse if they have a serious psychiatric or medical condition? 

This is the thing I'm really confused about, and is probably what's kept me staying for so long in an abusive relationship, because I keep making excuses for him. He has multiple mental health conditions, and possible a neurological disorder as well. So I've been excusing/forgiving all the terrible ways he's treated me, thinking that he is not mentally/neurologiclaly well, but I still love him regardless (when he's nice, he's very sweet, but he has extreme rage and anger issues that can be cruel and terrifying).

So -- if they have a serious mental health condition, is it still abuse?
What about a physical/neurological condition?

If they act violently if they have a personality disorder, autism, chronic pain/illness, schizophrenia, dementia, Huntington's disease, etc...is it still considered "abuse", or is it just violent/unsafe behavior?

At what point should you stay with the person no matter what (even if they sometimes scare or endanger you) out of love and loyalty, vs prioritizing yourself/your own safety by leaving them?