Why are we tolerant and nice to those that are most cruel?
The answer that comes flowing through: Because, sometimes, we feel what others feel. And as a survival tactic, we do not want to feel more pain and sadness than we already do. Fearing further abandonment from the safety and love of which we are worthy might completely paralyze us.
Instead, take action to know the truth. Take action to halt the cruelty for the action is acute and the sadness is imaginary.
Most likely the cruel someone, is unable to feel compassion or any kind of sadness for what they inflict upon others. Embrace the long term and know that is where the safety and love is given and received by all.