@derek Denethor is a classic example, but a steward doesn't even have to prioritize self to be unfaithful to their charge. They just need to prioritize their own idea of what is good over their lord's.
I seem to recall C S Lewis having something to say about the dangers of being oppressed by a person who's convinced they're doing what's best for you, because they will never give up. Something similar could be said of stewards who are convinced they're looking after their lord's best interests.