A troll always have an agenda, a goal or a motive. Something that drives the troll. This is very apparent when the troll hijacks threads or when he is creating antagonizing threads.
Without the agenda, the troll has no purpose. The Agenda is not static, however, and evolves along the way when the troll finds new objective, enemies or issues to incorporate into the Agenda.
The Agenda is most prominent when the troll posts about apparently benign things only to expose his intentions later on. An example of this kind game can be found at Michael Digest, Gibberish and Michael Digest, email.pdf edition