When Sirius Black breaks out of Azkaban and visits Harry, he sees how his relatives were treating him, and he kills them.

Sirius Black didn’t kill the Dursleys because he enjoyed it. He didn’t do it because he was cruel. He did it because he saw that Harry was hungry and scared and desperate. There was nobody else to help him, so he did something about it himself. He was already accused of being a mass murderer. Why not commit the crime he was imprisoned for. Harry Potter and Sirius Black were broken, but the world was broken first.