That's an almost impossibly broad question to answer.
In the context of this conversation? Moderation is evil because it is destructive, repressive, exploitable for the cause of tyranny (eg silencing dissent) and is the opposite of that which is good (freedom of expression, freedom of thought, free flow of information).
Like any other violence, there are times when it's warrented but it's always a balance. The greater good is to keep the forum open, so it's necessary to remove illegal materials. And that's an easy call when what is illegal is child pornography; but what about when what is illegal is a political message or even just a series of seemingly random numbers?
Sometimes it's the lesser evil (child porn is always the greater evil) and sometimes moderation is the greater evil (repressing political messages or being used in the service of propping up totalitarian regimes).
I apologize if this doesn't make things clearer for you. As I said at the beginning; the nature of evil is a broad question that has been wrestled by philosophers for millennia.
