merciless
unmerciful
bowelless
callous
cruel
dog-eat-dog
flinty
griefless
hard
harsh
heartless
impenitent
implacable
inclement
inexorable
pitiless
regretless
relentless
ruthless
savage
shameless
sorrowless
unashamed
uncompassionate
uncompassioned
uncontrite
unfeeling
unforgiving
ungrieving
unpitiful
unpitying
unregretful
unregretting
unrelenting
unremitting
unremorseful
unrepentant
unrepining
unrueful
unsorrowful
unsorrowing
bloodthirsty
cold-blooded
ferocious
fiendish
indomitable
inflexible
inhuman
insatiable
resistless
sanguinary
tireless
unsparing
vindictive