As C. S. Lewis put it in Screwtape: "To be greatly and effectively wicked a man needs some virtue. What would Attila have been without his courage, or Shylock without self-denial as regards the flesh?"
I was thinking of the houses in terms of the four cardinal virtues -- it would be easier to do the seven lively virtues, because then Slytherin would clearly be Zeal, the opposite of the deadly sin of Sloth, which is one of the commonest villainous virtues. (And the opposite one of the commonest heroic flaws. Much of the appeal of villain is often this.)
However, there are, of course, seven of them, and given four houses. . . .
I was thinking that it would be courage to Gryffindor, prudence to Ravenclaw, temperance to Hufflepuff, which would leave justice to Slytherin. Which does not work at all. Then I was reading Goblet and Cedric's argument with Harry, trying to get him to accept the victory because of what Harry had done. And come to think of it, Hufflepuff takes in everyone lacking the striking traits the other houses are looking for, so that no wizardling goes untrained, which is certainly rendering every man his due.
So, hmm, Slytherin as temperance. . . one has to remember that it means self-control, not an intemperate denunciation of temperate drinking, but certainly, to succeed in Slytherin House, one would need it.