Vigilantism is the seeming motivation for the "Purity" killers in Purity in Death. They've developed a virus that can be passed on from computer to human via email, making their victims increasingly sick and miserable until they take their own lives. Of course, it also makes them violent if they're interrupted while they're sick, which is why the collateral damage from their attacks are so high (intentional kills: 4, innocent victims killed: 3, innocents injured: 3 total/two serious). The collateral damage is especially meaningful to the readers since it involves two characters we know and love -- Officer Troy Truehart, who is injured by (and forced to kill) the first Purity victim, and Detective Ian McNabb, who spends much of the book wondering if he'll ever walk again.

This would have been a better book if the villains hadn't been so mustache-twirlingly evil. Would have been a better philosophical argument, too, in that case, but overall it's still a great book.