Yeah and if his niece was a guy it would be ok ?
Sorry, but when someone uses violence against me, like lets say, beats me with a bat on the head, i don't give shit if it's a man, woman, young, old or some estranged family member...
You arer portraying the MC as if he just randomly beat up his poor, inocent, kid niece, which did nothing wrong.
They are family, but he didn't see her for a long time, apparently because she didn't wanted. To be family means more than just being related and a family member, which don't want to have contact with u for years, is not much better than a stranger.
That poor adult niece, which leads a 'gang', is involved with the mob and first thing she does is beating him on the head with a bat, don't even apologizes besides, 'whoops wrong person'... and after that just continues as if he wronged her somewhere.
Then for some fucking reason, she wants to unleash his inner beast (cause MC is somehow Dr. Jekyll und Mr. Hyde all of sudden), which she somehow sees in his eyes, same as she knows him fucking young girls, dunno maybe some superpowered precognition on her part...
So she does everything possible to provoke him, find a way so he will lose it and will punch her, this is exatly what she wanted to achieve...so congratulations goal achieved.
We all have a point where we will snap, admittedly the MC looses it extremly fast, even more so as he is extremely level headed and in control otherwise, dunno maybe he was still a bit pissed at the hit on his head and her don't giving a fuck...
But if u find that so 'repulsive' than maybe look at the damn niece, she has a major part of blame in that.
Maybe, just maybe, don't intentionally provoke someone to the point where he/she will lose it...cause u know, they will lose control and lash out, often physically...if u do that intentionally, than look in the mirror, if u want to find the main culprit to put the blame on, being a woman doesn't change that