Synonyms for Merciless (same or very similar meaning)
WordNet sense 1 (without mercy or pity):
pitiless, remorseless, ruthlessWordNet sense 2 (unrelenting and deadly):
mortalWordNet sense 3 (ruthless in competition):
fierceWordNet sense 4 (having or showing no mercy):
unmercifulFrom the ODE community, based on WordNetadd/edit