It almost seems that the father has a real reason to do what he does, since he doesn't appreciate his son's crazy sadism. who knows what is the secret of that family
I always feels afraid to see what would happened to the boy the MC has crushed on in the new chapter. I feels like there's always a new possibility where he would die early.
Probably, the father genuinly believes in the religion but doesn't enjoy the killing, so he doesn't like the sons, who do enjoy the killing part. The mother is probably a psycho as well.
So I guess the question in the end is, is their religion thing real? Are their horrific blood sacrifices actually doing something important or are they just a bunch of pathetic homicidal loonies?
Knowing Uguisu's works the Father likely has legitimate reasons for doing those especially considering he doesn't choose the more sadistic children he has and chose Mari instead. I won't be surprised they're sealing somekind of eldritch monster there that requires sacrifices regularly.