Most of the monsters that humanity has created in mythology reflect the deep dread we have of our innermost workings. We know what our most base instincts are and we try not to look at them but rather to project our fear outward to monsters that are exterior to ourselves.
Mankind has two things that it has claimed for itself - consciousness and conscience. These things have given us what we believe a moral claim over the rest of the world, a declaration that it is ours because although we are not on top of the food chain, we have the ability to invent weapons of destruction that will make us so. It has turned us into the most frightening monster.
I believe the fact that mankind creates monsters does, in some ways, show that we are quite frightening. When you consider the tactics people have to use to get ahead in this world, we are a gruesome breed of creature that this planet has never seen before. Not to mention we even created weapons that could wipe us all off the face of the planet.
Which you could also call a social abstraction. Calling humans monsters is completely counterproductive and pointless. Our social norms and values should always be used to aid our species in its existence and success, not against it. They are all subjective ultimately, so the only logical way to use them is to enhance ourselves individually and collectively. Labeling humans as a whole as "monsters" defeats the very purpose of having socially condemnatory labels.
"Monster" is just a social abstraction. It does not refer to anything concrete. Social abstractions function according to their purpose in society. For instance we may call a serial killer or rapist a monster in condemnation of his deeds.
Hence using the label "monster" is any way that goes against the best interests of our species is self-defeating and pointless. The ability of humans to form social norms and have morality should always be towards the good of our own species and not turned against ourselves.
Even though I think this question has a good point, I really do not feel there is not a "most frightening" monster. To me, if I had to give anyone that particular title, I would have to give it to Satan himself as the most frightening monster in the whole world.
No, the fact that mankind creates monsters does not prove that mankind is the most frightening monster of all, because the monsters of mankind are just a few people who have decided to be terrible. Not all people who commit crimes are representative of their entire species. Humans on the whole are generally kind, loving people.