inhuman / inhumane
Not human, or monstrous, versus cruel.
Inhuman means not human, or monstrous in degree: inhuman strength, inhuman cruelty. Inhumane means cruel, lacking mercy: inhumane treatment. For ordinary cruelty, inhumane is the word.
Examples
Useinhuman strength
Useinhumane treatment
Avoidinhuman treatment (If you mean only that the treatment was cruel.)