What does it mean to lose your humanity?

It means you no longer hold any value toward human life and/or human beings. You don't care about what happens to anything or anyone, including yourself.

It Means the loss of your existence. Your loss of value to others existence. It is often compared with the loss of emotions or the becoming of a demon as losing emotions usually means the loss of sympathy. But in truth the loss of humanity is the loss of the belief that many humans run on; Ethics mainly as well as sympathy it is a very open ended phrase.