I believe a troll wants to ruin others' fun just for that reason. I find it abhorrent behaviour.
I remember reading an article about ganking a few years ago where a psychology professor (and gamer in his free time) speculated -based on his experiences- that it takes a very peculiar mindset to enjoy ganking others (the 'Dark Tetrad'/'Dark Triad' the article mentions).
In his words (as close as I can remember them), there are the "psychologically unremarkable" people who sometimes engage in ganking - but they do so because they have a temporary cause. For example, they are angry because of something that happened outside of the game, and want to blow off steam. They feel miserable, and they want to "share the feeling" in a crab-bucket kind of way: If I make others miserable, I am alleviating my own misery.
But then there are people who genuinely derive pleasure from torturing and/or manipulating others, often in conjunction with getting attention from an audience. And they seem to gravitate to certain kinds of gaming activities and settings. Such as ganking in an OpenPvP setting. Or trolling fora, as it seems.
That reminds me: A few years back, there were a lot of "apologists" in games and on fora I frequented that excused such behaviour with the statement that "this is not real world, people behave differently in games".
They seemed to genuinely believe that personality structure was so malleable that it was absolutely influenced (to the point of 180° reversals) by your surroundings and current situation.
I think that games and the internet in particular have an dis-inhibiting effect on many people that accentuates personality traits - they do not change, but are more visible.