The MC considers how he might have stopped this by never accepting Big Brother's hand, and then toward the end he thinks he should have just stood up for himself
though that ends up not working for him
The one thing he doesn't consider though is actually treating Big Brother like a brother. BB keeps offering to hang out with him but he always (understandably) wants to avoid him after he's committed violence, but what if he had actually hung out with him?
The why (never mind the how) is never really even touched on in this story, which on one hand deepens the terror, but also makes it feel a little hollow for me.