The adjective inhumane means cruel and heartless. Use it to describe terrible things like your neighbor's inhumane treatment of his hound dogs, which he leaves tied up in the yard in all kinds of weather.
Whenever someone acts without considering the discomfort or pain of another person or animal, you can describe their actions as inhumane. A cruel government that keeps prisoners in terrible conditions is inhumane, and the treatment of farm animals is in some cases revealed to be inhumane by investigators. The word inhumane was originally a synonym of inhuman, literally "not human," but it fell out of use and then was revived in the 1820s to mean the opposite of humane.