Responses
There's also the fact that the younger brother mentioned that he wanted to kill the dad (even if he didn't mean it). The dad was no context given to the statement (younger brother felt it was heartbreaking that older brother's life revolved around waiting for someone that younger brother believed to be dead) and that probably pushed the dad over the edge since he had already guessed he had sex with his son and was overcome with more guilt on top of abandoning his family.
I think the dad killed himself both out of guilt of having had sex with his son and he didnt want the younger bro to leave the other behind because he had left them there and was irresponsible, so he thought he wasnt suitable to be a dad (or a moral human being since he just did his son)
But it really hurts to see everyone slutchaming the older bro. Like he was raped and sold by his foster father thats absolutely terrible (and like others said, he likely sold himself after because thats how he can help his little bro survive) i think everyone is more adapted to the light hearted mangas and this is deeper into the mature and psychological genres