Inhuman means "cruel," "brutal," "lacking normal human compassion, pity, sympathy, and kindness": "Treatment of prisoners in that jail was considered inhuman." "Nero is usually described as having been an inhuman Emperor." Unhuman, a much less common word, is occasionally used, somewhat loosely, in the above-mentioned meanings of inhuman. Careful use confines unhuman to the meaning of "not human," "without the characteristics and attributes of human beings": "The sounds issuing from the wounded man's throat were unhuman." "To make mistakes is human; never to make a mistake is unhuman."
Dictionary of problem words and expressions. Harry Shaw. 1975.