4chan users are not trolls. They're griefers and sociopaths. Like you said, trolls are generally part of a community, or make an attempt to get a rise out of people through détournement, not through outright antagonism.

Over at kuro5hin.org we experienced a 'trollocaust' as it were, wherein the trolls successfully drove all the emotional beautiful souls off to another site (hulver.com). If the critical mass of trolls grows too large, community can't really be sustained.