Synonyms for Pitiless (same or very similar meaning)
WordNet sense 1 (without mercy or pity):
remorseless, ruthless, unpityingWordNet sense 2 (having or showing no mercy):
merciless, unmercifulWordNet sense 3 (deficient in humane and kindly feelings):
unkindWordNet sense 4 (lacking and reflecting lack of pity or compassion):
inhumaneFrom the ODE community, based on WordNetadd/edit