Idea: Vampires soak up evil

Vampires collect… Sin. Evil. Depravity. Corruption. There is something that is horrible beyond imagination, something which incites others to commit unsavory acts, inspiring them to lie or steal small things in small amounts, and leading nations down the path of genocide when it is too widespread. People can be horrible without its influence, but it certainly doesn’t make it easier to choose the right course.

Just by being there, however, vampires act like sponges for this force. While the acts of humans causes this force to spread, vampires do not cause it to do so, no matter how vile they act, and in fact reduce its presence in the area. They slowly become more vicious and inhumane as time goes on, as more corruptive force is gathered inside them and ferments into a more potent substance, but they still do nothing but decrease the overall amount of horror in their territories. The older they are, the more they hold inside themselves, and the more potent it becomes. When slain, it is left to remain in the vicinity of where the vampire died, and there it stays for a short time before fading away. Before that happens, however, someone else might come across it, and be affected… negatively.

Vampire hunters often do far more harm than good, even when killing, especially when killing, thousand-year-old monsters who do not conceive of humans as anything more than cattle. 

tl;dr vampires are Johnny the Homicidal Maniac