Firstly, if you leave your girl so she can continue to get beat you're a shitbag.

Leaving to get the gun isn’t being a shitbag.

Secondly, yes you can be charged for shooting someone whos hitting your girlfriend.

You could be and you could have an affirmative defense. It depends on the circumstances.

There is no law. No. Law. That allows you to escape from danger or leave the scene of a crime and return with a gun and shoot the guy. You have left. You're no longer party to the situation

This is where you are wrong. There are both federal and state laws that allow a person to use deadly force to stop a crime including ones that are not party to. What law separates me being able to hit someone who is hitting my wife but not if I am in the other room. How far do I got to be away under your theory for it to be illegal for me to intervene? You don’t even have to have been there to begin with. In my state if you come upon a guy setting fire to an occupied dwelling even though you are at no risk you can intervene with force. You can intervene in a rape with force and it makes no difference is you were never at risk or if you ducked back into your apartment to get a baseball bat. There is no law that says if I am safe and have already retreated that I can’t return to defend others or affect a citizens arrest and use force to do so.