I reached out to my childhood bully anonymously. Was this a bad idea?

No, I don't think it was, actually, it could be a great way to settle up old issues that definitely affected our personalities at the present.

I think wanting to reach out to someone who bullied you as a child is a way to overcome the traumatic experience that you had to face while growing up. I do not think that it is a bad idea to reach out to someone that bullied you to express how you felt about it and how it changed you as a person. Doing this action could help move on and not have any resentment towards that individual and can overall make you a happier individual. As long as the intentions are pure and genuine.