Wiesel is saying that if you are not part of the solution, you are part of the problem. When people are oppressed (or victimized, or bullied, etc.), if you stand by and say/do nothing, you are just as bad as the oppressor. Even though you may feel bad about what is happening, if you DO nothing, it is the same as agreeing with the situation.
People of integrity take action when they witness injustice.