Why is my brother like this?

I am a girl who is currently 24 years old. I am just an average American girl who is currently trying to finish college.

Here is the main part of my post:

Back in highschool, my brother was bullied by the entire school to the point that only a few popular students protected him.

The rest of the other popular students lead the bullying against him but how come that now that he has finished highschool a long time ago, he forgave the popular bullies that bullied him but not the average students that joined in the bullying?

He didn't just reject the average students but also hated them. He is also very hostile to them.

Why is this? Why did he favor the popular students over the average students?
Why is my brother like this?
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