Oh! He knew this one. Or rather, he knew it in his own universe.

"Well," he said, puffing up at the ability to disclose new information. "Where I come from, a vampire's aggressive nature is created by his lack of a soul. They function as a moral compass, so to speak, and vampires are notably devoid of them. I'm only familiar with one vampire who managed to re-ensoul himself. Prior to that, he was among the most vicious and destructive vampires in existence, but once ensouled, he became much more passive. Moral, even."

Watchers were big on ghost stories. It was kind of a thing.