What possesses people to hurt kids, especially their own?!?!?

my step fater beat me for 13 years my lovin mother left the room each time, only returning to take me to hospital for the latest stitches, i was taken off them when I was 4, he continued hurting me till I was 13, shes dead, she was no mother to me, and hes still alive sadly i see him in town we dont speak, hes a B*****d