General Synonyms:
* Merciless
* Cruel
* Heartless
* Unfeeling
* Hard-hearted
* Cold-blooded
* Unforgiving
* Relentless
* Inhuman
More Specific Synonyms:
* Callous: Emphasizes a lack of empathy or concern for others' suffering.
* Savage: Implies a brutal and violent nature.
* Despicable: Expresses strong moral disapproval.
* Vicious: Highlights a malicious and aggressive nature.
* Tyrannical: Implies oppressive and cruel behavior, often associated with power.
The best synonym will depend on the specific context and the emotion you want to express.