For the trolls, the internet is a playground but also a laboratory. There they can experiment with language and its effects, with norms, taboos, their limits and what happens if you break them. And you can do this in a safe and practically consequence-free environment.
The freedom must be exhilarating. They can go onto memorial sites for children who have died of cancer and say the most horrible things, and no-one can catch them. There seems no price to pay for provoking some guy you’ll never meet. They are never going to get you in a headlock and punch the shit out of you. You’ll never have to catch their reproachful, wounded eye across the room. Meanwhile the sheer number of conflicts you can indulge in makes up for the relative mildness of the emotional rewards.
Maybe this appeals most to isolated people, those who find it difficult to form meaningful relationships, perhaps they are numb, or have stunted social antennae and need the strongest emotional responses – anger, pain – to make a human connection, to be sure they are communicating, that they are having an impact on others.