A troll is a person who, often through malice, deliberately attacks an Internet community in order to cause discord there.
On an ex-Mormon bulletin board, for example, you will often find Mormons pretending to be ex-Mormons or non-Mormons.
Often they will band together to single out one particular poster to try to hurt and destroy that poster. They use the tag team technique. It is suspected that they use email and or phone communications before, during and after their attacks in order to better plan them.
Why do they do this? Quite simply, it is because they can. They see former-Mormons or questioning Mormons as "FAIR game" and see it as a god-given duty to destroy them or force them back to the fold as broken people.
They will lie for the lord, because they see lying and being cruel and hurtful as being sanctified by God.
This type of "morality lite" Mormon troll is especially dangerous, as it is likely that In Real Life (IRL) that they may present a danger to their local community as they show signs of severe sociopathic mental health issues. If untreated it is not beyond the bounds of possibility that some of them may "go postal" with horrible consequences.
Other Mormon trolls will ask nonsense questions, attempt to derail threads or will pretend to be a legitimate poster with a problem, only to return to the board and boast about how they tricked everyone who took the time to express sympathy or offer advice.
This type of troll is a danger to the Mormon church as they really only hurt it.