The mother abused mentally, emotionally to him. When he discovered that the 7th reason why he wouldn't go outside was because of the mother. So when he saw the mother on top of him and heard everything, he beat her, basically, and then got to jail. And his brother is waiting him "outside" finally.
Ooohhhh now I get it! Thanks!
Also if I'm not woring it was kinda implied she sexually abused tooru too, especially with the way you can see how her breasts were visible. It makes the situation even worse and probably the more reason why satoru went into rage and beat their mom unconscious
Can someone explain to me what happened? Cuz my mind doesnt have any braincells left to think and analyze the situation.