Zhenya is antisocial but that doesn't mean he doesn't have any moral standards, they are just unique to his way of thinking.
First off, he doesn't really believe strong violence against women to be necessary and sees it as a bigger nuisance if they end up dead. In Olga's case for example, his entire family would turn oh him.
Also, Olga genuinely cares about him and despite his annoyance toward her he somewhat respects her as an ally against their family. She is also terminally ill and could die at any moment, so it seems he doesn't insist on shortening any more of her lifespan knowing she could never be a serious threat anyway. I guess you could call this some semblance of familiar love. I mean they do act like squabbling siblings at times even if their squabbling is somewhat unusual.
To clarify: he actually does hurt her on multiple occasions, but he is significantly easier on her than anyone else

I read some parts if the novel so I know what happens next. But does anyone know why zhenya never hurts olga or like chooses not to despite his anger issues