Josh Marshall shares this extraordinarily sharp passage from John-Paul Sartre's Anti-Semite and Jew, which seems to capture the essence of trolling. Trolls understand the asymmetry of reasonableness. They understand that when your opponents feel constrained by the need to engage seriously and responsibly, the willingness to be ridiculous and offensive can provide a great advantage.
“Never believe that anti-Semites are completely unaware of the absurdity of their replies. They know that their remarks are frivolous, open to challenge. But they are amusing themselves, for it is their adversary who is obliged to use words responsibly, since he believes in words. The anti-Semites have the right to play. They even like to play with discourse for, by giving ridiculous reasons, they discredit the seriousness of their interlocutors. They delight in acting in bad faith, since they seek not to persuade by sound argument but to intimidate and disconcert. If you press them too closely, they will abruptly fall silent, loftily indicating by some phrase that the time for argument is past.”