1) Bitter 2) Implacable 3) Merciless 4) Pitiless 5) Rancorous 6) Resentful 7) Revengeful 8) Ruthless 9) Unrelenting 10) Vengeful 11) Vindictive 1) Carrying a grudge 2) Cruel 3) Grim 4) Implacable 5) Inexorable 6) Meanspirited 7) Merciless 8) Pitiless 9) Relentless 10) Revengeful 11) Rigid 12) Strict 13) Unappeasable 14) Uncharitable 15) Ungenerous 16) Unrelenting 17) Vengeful 18) Vindictive 
[adj] - unwilling or unable to forgive or show mercy
adjective unwilling or unable to forgive or show mercy; `a surly unforgiving old woman`
not disposed to forgive or show mercy; unrelenting. · not allowing for mistakes, carelessness, or weakness: the unforgiving nature of aviation.