The antonyms for the word "hadst mercy" are cruelty, harshness, and ruthlessness. When someone shows no mercy, they are being cruel and showing no compassion. Harshness is also an antonym as it implies being uncompromising and unsympathetic. Similarly, ruthlessness is a word used to describe someone who is without pity or compassion. These words are used to depict a person's character or actions, which can harm others' well-being. Therefore, it is crucial to display kindness, empathy, and sympathy towards others, regardless of their background or circumstances, for a more harmonious society.