Why is my girlfriend so mad at me?

So like Yesterday i was wit my girlfriend at the park just taking a walk when this Punk comes out off nowhere and pretends to also be taking a walk wen all off a sudden i hear her shriek i asked her wat was wrong and she told me that the guy that just ped us grabbed her *** ( NOT ACCIDENTALY BUT WITH FORCE) So i Go up to the dood and start beating the **** out off him and i force him 2 apologize in wich he does and runs off but then she was like OMG U DIDNT HAVE TO BEAT HIM UP THAT BAD and now shes mad,,Im like WTF I Just beat up a dood who just ualy ualted u in which in turn maybe i couldve been beat up and now shes mad at me?? I mean wat about u girls Would u be mad at me?? Or happy? And how can i tall her to cool off?