The Devil stands for everything that is concerned with excess and falsehood. He means lie and betrayal, (self)delusion and deceitfulness. He stands for crapulence, uncontrolled passions and lewdness, for aggression, ignorance, wastage, laziness and bragging. He basically means that something is out of balance. He can mean addiction and bondage, seduction, blackmailing and misuse of power, and he stands for treason and for not sticking to one's principles.
In very few cases I do read the Devil in a positive way. I do this if the person I read the cards for is passionless, is hostile against their body and its needs, or is morally overly correct to the point of harming themselves. In these rare cases I read the Devil as an invitation to enjoy (physical) life for a change, to feast, not to feel guilty for the tiniest moral transgression. Such positive readings of the Devil should stay an exception, though.