It's not an either/or situation. He is guilty AND it was peer pressure. The peer pressure is an explanation, not an excuse, for what he did. Many -- if not most -- bullies have some sort of motivation for what they do besides just 'I like being evil, heh heh'. For instance, in my school, the people who picked on me were often kids who weren't popular and got bullied themselves. One reason they ended up bullying other kids because they were insecure and they wanted to deflect attention off of themselves. They sucked for being bullies and they were totally responsible for their actions, but, in hindsight, I can also kind of understand and feel sympathy for them.

Humans are complex and most of the time they're not either good or bad. We can hold them culpable for the things they do wrong while simultaneously trying to understand their motivations.