I think Slughorn is a kind of vampire in that he takrs 'value' from those around him for his own benefit. For example, what sort of kickback did he get for choosing that dodgy textbook? Who is he likely to recommend for a post at Saint Mungo's? The better student or the one with an Aunt on the Wizengamot? How do you feel about the better connected but less effective healer treating your child? (What if s/he's out on the magical equivalent of the golf course?) For some reason that kind of predation is often more acceptable than someone thieving in order to eat.