Synonyms of callously

2 Syllables
coldly, meanly
3 Syllables
brutally, fiercely, unkindly, ruthlessly, viciously, brutishly, ferally, heartlessly
4 Syllables
unfeelingly, coldheartedly, murderously, ferociously, barbarously, relentlessly, savagely, mercilessly, atrociously, hardheartedly, inhumanly, pitilessly
5 Syllables
unrelentingly, inexorably, barbarically, demoniacally, diabolically, inhumanely, remorselessly