They are a group of vampires that have assimilated into the general population. They have all made a pact that they DO NOT hunt humans. The one kid went astray, and violated their policy.

I recently watched this episode and I thought the same thing. But, if you remember, Sheriff Hartwell tells Scully that nowadays vampires live normal lives; they pay taxes, are good neighbors, etc. Based on that, I would think that killing two federal agents is something that the vampires would want to avoid because it would cause even more heat to be put on them than if they simply let the agents live and tell their story. After all, who would believe them--right. . . ? ; )