The phrase "showing no mercy" refers to an intentional lack of compassion or empathy towards someone or something. Synonyms for this phrase may include words such as pitiless, cruel, merciless, unrelenting, unforgiving, and heartless. Each of these words highlights the idea of a complete absence of mercy or kindness, and implies a willingness to cause harm or distress without regard for the consequences. Whether being used to describe an individual or a group, the use of these synonyms suggests a dangerous or harmful approach that may result in significant harm or suffering.