...and before someone draws parallels to terrorists or suicide bombers...
Many of them are performing their act of terror under the pretense that there is some reward...just in the afterlife. Ledger's Joker was almost monster-like, an unstoppable force of evil that is almost impossible to combat.
The type of villain that makes you go, "hey...that guy is scary."
Had they not fucked up Spiderman with the awful Spiderman 3, I would have liked to have seen the appearance of Carnage. Carnage is a brutal serial killer, he could have easily rivaled the destruction of the Joker.