Webster's 1828 Dictionary
UNMERCIFULLY
adv. Without mercy or tenderness; cruelly.
American Oxford Thesaurus
unmercifully
unmercifully adjective she treated her poor little nephews unmercifully: ruthlessly, cruelly, harshly, mercilessly, pitilessly, cold-bloodedly, hard-heartedly, callously, brutally, severely, unforgivingly, inhumanely, inhumanly, heartlessly, unsympathetically, unfeelingly, unsparingly.
Sanseido Wisdom Dictionary
unmercifully
ù n m é r ci ful ly 副詞 残酷に, 情け容赦もなく .