why do some people who dont believe in God, waste their time insulting people that do?

Answer 1

I know that many of them think believers only believe because they have been indoctrinated or brainwashed. Since they see not valid reason why anyone should believe in God, they find it their obligation to enlighten the believers.

Answer 2

I'm sure there is many reason, but my guess, they feel their doing the same thing that believers are do. Such as ministering that there isn't a god. Or its their reaction to pressures of guilt be pushed on them, either necessary or not. I really don't know actually, ask them why next time.