Please, that guy is a low-life. It wasn't his violent nature that attracted her in the first place. Everyone goes through the same dating process where both parties put on their best behavior at all times. You think she would have proceed to become his gf if he slapped her during the first date?
After they are attached, he became the abuser and she, the enabler. She enabled his violence. It takes two hands to clap. I won't disagree that there's a subset of women who will stay attracted & loyal to such men, but the majority of women (at least those with dignity) wouldn't.
Everyone is Mr and Mrs Nice during the initial phase. It's how you act and behave at the later stages that separates you between an upstanding man and a low-life.