Cruelty is the wilful infliction of unnecessary pain and suffering, i.e. pain and suffering that are not necessary to achieve morally important ends and thus cannot be justified by those ends. Cruelty is the infliction of pain and suffering for their own sake on any sentient creature that can experience pain and undergo suffering. It can be physical, emotional or spiritual. It can be directed toward humans or other animals and, some would suggest, toward plants. It is destructive toward both the perpetrator and the victim.
I cruelly hate cruelty, both by nature and judgement, as the worst of all vices. (Montaigne).