Just a few reasons to start off with:
1: We can get along under the freezing point of water anywhere up to halfway to the boiling point of water. While naked.
2: When you cut us, it regenerates.
3: We can remain functional even after losing a limb. Or three. Or four. Some members of human society are still influential even though they're almost completely paralysed.
4: We have at least 5 senses, not counting internalised senses like balance and proprioception
5: We think it's a good idea to go to war over abstract concepts. Seriously. What species will go fight for peace?
6: Sometimes, in times of stress, we can display extraordinary feats of strength, endurance or outright inability to feel pain. Some humans hack off their own limbs in order to survive, while fully conscious and cognisant of what they're doing.
7: Some of us just go ahead and permanently punch holes through our bodies anyway, for decoration.
8: It isn't uncommon for us to strap ourselves into metal boxes filled with explosive material and make them go very fast around tight tracks. This is considered 'fun'
9: We not only farm animals, we've optimised the process for meat production to the point where we give life to, grow and kill billions of life forms a year. Much of this meat goes to waste.
10: One of us came up with the concept of Mutually Assured Destruction. We gave them a prize. For promoting Peace.
There are more, but hopefully this list gives some flavour of how bat-bleep terrifying humanity is.