This seems like a somewhat vague question. By historians or by the people they ruled over? To put (democratically elected) presidents and dictators in the same category also seems somewhat odd, as does making blanket statements about all kings and all dictators.

Do you have a specific example in mind?

OP, something that you have to realize first is that a king is not a dictator. The title of "king", or "emperor" for that matter carries certain connotations not included in the meaning of dictator, and they add "legitimacy" to the position that the position of a pure dictator does not have. A monarch can be called a tyrant, but dictator is a different political classification.

Assured succession, divine right of kings, the fact their ancestor actually conquered the land at some point in history and didn't just commit a political coup, etc...