Help with finding a manga

The MC was a bright and positive kid, with an amazing family
But he gets targeted by a group of bullies. The ringleader is a sociopathic girl who wants to see what happens if you take everything away from a person. She basically treats him as an experiment

It eventually escalates to the gang causing MC’s family to get into a car accident which kills his family, leaving MC alone

Finally MC breaks and wants to get revenge and kill them, but his grandfather (?) who was a war veteran stops him and ask him if he’ll be satisfied with that or does he want them to suffer and beg for death

MC eventually begins his revenge mission a few years later, where he just takes in the bullying, pretending to be weak but he’s hiding his strength and experimenting