Without mercy means showing no compassion or forgiveness towards someone. There are a variety of synonyms for this phrase such as relentlessly, ruthlessly, pitilessly, cold-bloodedly, callously, heartlessly, and mercilessly. Using any of these synonyms of "without mercy" gives a clear indication of a lack of empathy and an absence of kindness. It implies that the person in question is willing to harm another without a second thought, and cannot be swayed by pleas for leniency or compassion. These synonyms are useful in literature, news stories or legal documents where the writer wishes to convey the harshness and severity of an action being taken against a person or group.