I'm mad at my girlfriend, so I'm going to punch her.
You will be hurting her if you do that, why would you punch her?
It is because of her aggressive nature. It is sometime important to treat people equally.
It is wrong to abuse your girlfriend. You should get help or break up if your relationship is not working out and violent.
Why? She loved it.
I do not think it's a healthy relationship if you will be constantly violent with each other and punching her, as it is a form of domestic abuse. If she also is aggressive towards you, you should tell her that you do not want that or at least report her to the authorities as physical assault is very illegal you should not commit to doing that with her.