Isn't the only difference between a dictatorship and a monarchy that a dictator has seized power by might of his own arms while a king's power is by virtue of one of his ancestors having seized power by might of his arms years ago? I don't think anyone would deny the possibility of a benignancy in a monarchy, so why not in a dictatorship?

There is the consideration that, since a dictator has seized power himself, he is apt to be more highly concerned than a monarch that someone else might try to seize that power from him. And since staying in power is, presumably, a dictator's primary concern, when a policy he has deemed necessary to insure continuation of his power runs contrary to the public good, the public is going to lose every time.

On the other hand, if a dictator's policies are such that he achieves a wide enough support base that usurpation of his power becomes unlikely, then why wouldn't he institute policies that work toward the public good?