The answer to your question is so that you can be purged into a better man. I have faced many of this kind of trial in school too but I got used to it along the way and it doesn't use to register anymore to me when it comes, in fact they got so acquaintanted with my way that they stopped sitting close to me during examinations.

It is thesame people who want good that are still sponsoring evil and complaining about how that things are not getting better.

Your response to the guy's action is perfect, ignoring such a person and the talks that might come forth from any other person is the best thing. There is honor in standing alone for what is right. I know you felt bad for not helping the guy, that is the work of human sympathy, Incase no one had told you this before, let me be the first to tell you; you did right, don't ever feel bad for doing what is supposed but always apply wisdom. I am glad you moved on