I remember my stepdad being violent growing up. My mom had black eyes on more than one occasion. So I have always had a real hatred for violence. That being said, I did find myself in a situation where I had to go against my instincts.
I was dating a girl and after 6 weeks or so began to notice how, well, unstable she was. I was over at her place and decided to end things with her. She flew into a rage, screaming, yelling, etc. When she stormed off into the kitchen, I figured it was a good time to leave. As I walk past the kitchen towards the door, she comes out with a knife in her hand and slashes at me. In the couple seconds that followed, I got cut a few times (6 years later, I've still got scars on my hands and chest) before I managed to get control of the situation. It ended when I was able to grab her wrist and shove her backwards, causing her to lose her footing and fall to the floor. At that point I got the hell outta Dodge.
I called up a good friend of mine that night because I was feeling bad about pushing the psycho. She told me "You should never strike a lady, but a woman who attacks you is no lady." That's my principle to this day. I won't throw the first punch, but if you hit me all bets are off. I will do whatever I believe is necessary to protect myself, gender be damned.