Cowards are always cruel.

The proverb presents cruelty as a characteristic of cowardice. It condemns someone who acts harshly while lacking the courage to behave otherwise.

Its absolute wording gives the judgment force. It is a pointed assessment of character, not a reliable claim that every person who feels fear will mistreat others.

Use it to criticize a person whose harsh treatment of others is seen as connected to a failure of courage. Because it judges character broadly, it suits a forceful rebuke better than a careful assessment of someone's motives.