I have starved, dissected, sewn together, pulled apart, burned, dissolved, and variously tested Demons as much as I could over my long life. 

It takes a keen eye to observe, but I have come to the conclusion that all the "emotions" and "humanity" that they display is entirely false. It is not even that they are lying or pretending, they are simply not even sentient beings.

They go through all the motions, and will even defend their own personhood vehemently, but ultimately they have as much sentience as a tree, or an apple. A clockwork plant, wound by a vile hand, and set on a path of destruction.

If only there was a way to not see them, hear them, or even perceive them.

-Notes on Demon Biology by The Highest of Angels