What was Machiavelli's opinion of mankind?
Throughout his writings in The Prince, Machiavelli discusses people in a rather scientific way, the idea being that if you take a certain action the people will respond a certain way. If you take another action, they will respond in a different way.
Much like many powerful people (in his day and throughout history), mankind is seen as predictable, as a sort of animal to be mastered and ruled over. Machiavelli's concern was how best to provide for the defense of a nation or city-state and he placed a great deal of importance on pleasing the populace. A ruler new to the throne must quickly take on any unpleasant cruel actions to consolidate their power but then should move quickly to show kindness and magnanimity to the people so that they develop a sense of loyalty.
His representation of mankind as a relatively simple and predictable entity is telling in that he does not give the people in his treatise the ability to think critically about a ruler's actions, rather they see a certain action they react predictably.