If you look at flashbacks Rize had with her brother it'd be littered with beer cans, and Masaki is mostly bruised around the face, Rize being afraid to come out of the futon saying bad things always happen when she leaves her futon, how her father didn't change even after she was born, etc. God knows what else happened when Masaki was left in the custody of their father, victims can become the abusers. Maybe during the years he was away from Rize he's become something that he feels can't meet Rize anymore. I don't know, maybe Kairi had put in some lies or exaggerated some things here and there.
Masaki abused, even implied(?) murdering their father, but I don't think he deserved to die, he needed help, Kairi does too. I think the best way still is to come clean to the police, though I don't think it'll be happening anytime soon, I foresee some kind of running from the police Sachiiro no One Room style, which is why I am interested to see Tachibana's role in this.
Thanks for the update CCS!