It seems that Gen has always been an onlooker (or 'bystander') while growing up. Witnessing Reiji's mother and Reiji himself being abused by her husband but not being able to do anything as a child can be deeply affecting. He feels partly at fault, partly responsible for his father's actions, and the rest of him just doesn't want to 'take the blame' anymore and just hide somewhere.
This chapter is Reiji's mother basically gaslighting him. By bringing him to the place where his father and her have sex, showing him how she's forced to dress in her school uniform- that's deeply traumatising for a 17 year old boy no matter how mature he presents himself.
Gen has always been put on a pedestal but he breaks down quite easily. We see him practically bullying Reiji and making him buy more cigarettes than he actually needs (seeing as he keeps all of them) just so he can still talk to him. Being raised by a shitty father, I'm not surprised Gen doesn't understand how to treat people with love. He's both a victim and a bully, it's actually really sad- we always see people looking up to him, fearing him, and calling him a 'hero'. Everyone has an ideal image and expects something from him whether bad or good. Reiji has shown different sides of himself to Nagi, Yuri, and Chako, but Gen has absolutely no one to turn to and break down in front of.
I hope he gets the help he needs and change for the better, but, that's kind of unlikely.