Perceived impunity, maybe? The divorcing of actions from their consequences? But you can say the same about school bullies, gaslighters, whatever: the notion's not unique to the internet. But it is amplified by it -- hyperbole to a ridiculous degree -- which makes its effects so much more damaging. At the same time, I disagree with Skepchick's summation that wanting power and wanting attention are mutually exclusive; giving the latter gives trolls the former, so never underestimate the power of turning the other cheek. There are other, more effective ways to undo them (though I know that's easy to say when I'm not at the receiving end of a barrage of hate).