There must be some kind of magic behind this. This short scene was so filled with meaning.

Why would the guy we know as a "good guy" bully these two? After he was shown protecting someone from bullies?

Because he can.

This showed me a perspective I had missed; people who do these things, are often portrayed as dirty and ugly in some way, or just evil. Often they are just inexplainably evil. Being sadistic and bored is often used as a reason.

But the real reason is ability.

An addict with ability will do whatever, simply because he can do it, and isn't stopped. It's not motivated by any specific reasoning, it's an animalistic energy with an outlet. Someone who doesn't have that outlet can't see this simple fact, and will get entangled in all sorts of moralistic and psychological reasoning.