So now we are already at question 22 of 30. Favorite Monster Ever. Somehow it's one of the easiest questions for me and the short of it would be:
Creative evil diversity with a high power curve. No monster in D&D is that powerful, intelligent and versatile without at the same time being the odd man in the crowd. I mean, powerful Monsters disguise themselves as humans to get along.
But cruelty is not bound to level, it's bound to motive. A simple level 1 maid can put a whole lot of social pressure on a group, if she were so inclined. Greed can be a very good motive, too. Just so many possibilities.
And there is not only raw individual power given by the rules and the story, there's also the power of the crowd. Agitated masses will ruin a player's day anytime. Humans are everywhere and if they weren't, they want to be there or have already been there some time ago.
The final reason would be that humans are the one most effective origin of other monsters. The lich, the vampire, a wizards evil aberrations are all but examples how far the influence off humans in a setting may reach. They are my Favorite Monster Ever.