Maybe becase you're a dick? She loved you and you completely betrayed her, crushed her heart, she felt like a little girl accidently letting go of a balloon. She grew close to you and absent mindedly, you let her whilst you were dirty enough to go shagging with your ex. How come i'm 14 years old and I have more sense than you?
She was hurt. Not saying it's right because in no way is it right at all. But you've disrespected her, made her feel unwanted, rejected, and have made her feel like she's less than the girl you cheated on her with. You guys were in a commitment together and you broke it. She's gonna be upset. And that's the way she reacted
Well wouldn't you be angry if the tables were turned? I maybe wouldn't go as far as beating the shit out of my boyfriend if he ever cheated on me. There's that saying, you don't know what you had until its gone, I don't think i could trust him enough to take him back. But yes, she sounds like she's a little too aggressive and violent, she could be doing with channeling that aggression on something else, thats coming from an ex kick boxer/kyu at judo. Dude I think she needs a punch bag. Lol
Bravo to her *claps* Um, you really haft to ask why. It's simple, she caught you cheating on her with your EX, her catching you cheating hurt her deep inside emotionally, so she physically hurt externally.
Why did you let her? Some women are and because they're a woman they think they can get away with it and usually will... and by "get away with it" I don't mean hitting her back or anything, I mean like pressing charges.