When did he stop? He literally punched her in the gut, that is assault man.
We should really get it straight what we are talking about. Lord Helmet was a little unspecific in ranting at me. There were two assaults in the game.
1. He was about to rape the woman, but he got himself under control befor doing any physical harm.
2. He punched the older woman in the gut, yeah, this he did do.
If you are talking about the second occurance: Yeah, I think this was not good. He overreacted. But I think this is also very believable. Try to get into his mind. He is abducted, he is under severe stress, he is powerless and helpless and his life is threatened, for days. Additionally, his sister comes into the same situation as he himself. He wants to protect her but is also completely powerless to do so.
And then he is asked to rape(!) his sister! Yes, rape, because she has never stated that she really wants to do it - she only says: "ok, before you die, you can do it with me!" So, from his perspective, he has to rape his sister. He is the raping tool that the person behind the camera uses. And the older woman is ok with this. She wants him to rape his sister in order to survive herself.
To summarize it: Rape is against the law, incest is and beating another one in the guts is too. But before the judge, his beating would likely go under self defence or better emergency defence for his sister.
So yeah, he could have tried to stop this in a more peaceful way, but they did not follow his demands either way. Even given his strong reaction and his loud and clear NO (that is supposed to mean no, right?), they do not accept his wishes.
Talking about saying no: Additionally, what they did to him also counts as (forced) rape. He did not want to have sex (with his sister) and he tried do defend himself against it.
So yeah, maybe, in this situation(!) I would also have punched her - even though I would not be proud about it. The girls might have realized that he only did it to protect his sister and himself. He is a soldier - in action, they even have to kill without hesitation in order to keep their camarades safe. He only chose one single punch, no much harm done. He could have killed her or broken her bones or worse... but he chose a relatively reasonable dosis of force. The was no police he could call to help him.