He didn't know how it turned into abuse. It started when Sirius backhanded the child for repeating his desire for something to eat. Severus just looked up at him with a miserable expression, but he didn't look surprised.
Sirius gave him food; not enough, but it was still food, and sent him to bed. The second time was about two hours later when the child whimpered-damn Dumbledore and his monitoring charms!- and awoke Sirius from his peaceful slumber.
Sirius barged into the small room, throwing the door open to hear Severus cry out.
"Stop! Please, I'll be good!" The boy cried in his sleep. That threw the recently cleared man into a frenzy.
"Snivellus! Shut up!" He screamed. The boy shot up from the bed into his arms- his dare he! Disgusted, Sirius threw Snape down, and when the boy tried to get up, he sent a vicious open-handed slap which sent the small child sprawling, kicking him in the ribs when he moved. Sirius stormed off to the kitchen for a stiff drink, leaving Severus curled up in a ball in the ground, weeping silently.
So the week dragged on. Sirius withheld dinner when the boy tried to go outside, and that night he was once again awakened by the monitoring charms.
Sirius crept into the kitchen. There was a crunching sound coming from around the corner. Sirius leapt around it and seized hold of Snape's shirt collar- the boy had snuck down to steal his food! In a fit of rage, he thrust the boy against the wall, strangling him. The boy fought, kicking his feet and twisting away, but stopped when Sirius drove his knee into his solar plexus. The convict only stopped when the boy was blue in the face. As soon as the boy weakly struggled to his feet, Sirius grabbed him by his dark, shoulder length hair and dragged him up the stairs. When he reached Snape's room, Sirius threw open the door and shoved Snape into it. He cast a locking charm on the door and strode for his bedroom to go to sleep.
The next day it was even worse. Dumbledore had packed Severus's wand in with the small amount of belongings, and somehow the boy already knew an unlocking charm. When Sirius woke up later that day- having slept through the monitoring spell's alert- he staggered to the bathroom to relieve himself. As he pulled off his pants and boxers, he turned to find the child swimming in an overflowing bath, happily splashing around.
"WHAT ARE YOU DOING!?" Sirius bellowed, his face darkening.
"I- I was trying to take a-"
"SHUT UP!" Sirius grabbed his belt and threw the boy over the bathtub ledge. He placed a sticking charm on the boy's torso and began to bring the thick leather belt down viciously on Severus's unprotected back. The boy didn't make a peep. Sirius folded the belt in half and brought it down with renewed vigor, harder with each stroke. After 20 on Snape's back, Sirius began to move towards the child's bottom and thighs, feeling his rage leave when the child began to cry out. When Sirius was through with him, his back and thighs were bleeding, and his bottom red and swollen. And then, the madness set in.
Sirius Black began to rape Severus Snape.