True trolling is about, among other things, plausible deniability. The troll identifies a pressure point in the collective unconscious of an online community, and then subtly attacks it. Depending on how skilled the troll is, what follows is a lot of attention and a large degree of over-reaction. The troll can then take the moral highground and pretend to be perplexed at these harsh responses, which of courses only provokes even more enraged responses. The cycle can be quite amusing. It can also generate/provoke lots of debate that a community would otherwise not be inclined to start.
It's not about personal attacks.