merciless

Fearful is a related term of merciless.

As adjectives the difference between fearful and merciless

is that fearful is frightening while merciless is showing no mercy; cruel and pitiless.

Enormous is a related term of merciless.

As adjectives the difference between enormous and merciless

is that enormous is (obsolete)  deviating from the norm; unusual, extraordinary while merciless is showing no mercy; cruel and pitiless.

Merciless is a related term of ungentle.

As adjectives the difference between merciless and ungentle

is that merciless is showing no mercy; cruel and pitiless while ungentle is not gentle.

Obdurate is a synonym of merciless.

As adjectives the difference between obdurate and merciless

is that obdurate is stubbornly persistent, generally in wrongdoing; refusing to reform or repent while merciless is showing no mercy; cruel and pitiless.

Merciless is a related term of satanic.

As adjectives the difference between merciless and satanic

is that merciless is showing no mercy; cruel and pitiless while satanic is satanic.

Monstrous is a related term of merciless.

As adjectives the difference between monstrous and merciless

is that monstrous is hideous or frightful while merciless is showing no mercy; cruel and pitiless.

Merciless is a related term of evil.

As adjectives the difference between merciless and evil

is that merciless is showing no mercy; cruel and pitiless while evil is intending to harm; malevolent.

As a noun evil is

moral badness; wickedness; malevolence; the forces or behaviors that are the opposite or enemy of good.

Sullen is a related term of merciless.

As adjectives the difference between sullen and merciless

is that sullen is having a brooding ill temper; sulky while merciless is showing no mercy; cruel and pitiless.

As a noun sullen

is (obsolete) one who is solitary, or lives alone; a hermit.

Merciless is a related term of hardened.

As adjectives the difference between merciless and hardened

is that merciless is showing no mercy; cruel and pitiless while hardened is unfeeling or lacking emotion due to experience; callous.

As a verb hardened is

(harden).

Cantankerous is a related term of merciless.

As adjectives the difference between cantankerous and merciless

is that cantankerous is given to or marked by an ill-tempered nature, ill-tempered, cranky, surly, crabby while merciless is showing no mercy; cruel and pitiless.