So how is him getting dicked down and loving it any type of atonement for his crimes? What...

Honestly it looks like self harm to me, his trauma is being sodomized by his father and « the rougher he is the lighter my heart become », look how he was crying at the start, not only that but he is also shown to be restrained and usually rapists hold down their victims so they cant run so i think since the thing he hated is being the same dogshit that is his father (generational trauma cycle yky) he feels like he should suffer like what he did to his lil bro but also that since growing up he was a victim too it wouldnt be wrong to say he feels like its his place and also feels guilty. When you grow up in pain you usually wanna stay in pain