There are big differences between thoughtless insensitivity and cruelty.   Thoughtlessness happens quite often, and it can be considered an accident when it does.  Mindfulness is rare, we live in a world of clashing, competing distractions.  Everyone is sometimes thoughtless.  The best of us apologize when our thoughtlessness hurts somebody else.

Cruelty is planned to inflict pain.  It is almost always done out of a sense of justice, but there is little justice in cruelty.  Cruelty is dreamed up by people who suffer, and it leads to more suffering.  Bad shit, cruelty.