When you love someone, often no matter what they do we tend to give endless chances, hope that things will get better, have the fantasy/belief they have (say during a "Honeymoon" cycle) or are not quite as bad as someone stepping over your body.
Though intelligently you know better, there's a lot of internal conversations going on, fear of the unknown, and by this time usually some pretty low self-esteem. you hate yourself for allowing the situation in the first place.
Even though you know there's pain, it's a a pain you're familiar with.
For me, I had to keep a box with all the police reports, diary, photos of a wall or windshield he'd punch, etc. so when I started to feel/hear that "possibility-of-hope" voice, right in front of me would be concrete evidence, facts, of what this person was really all about.