Black-hearted is a term used to describe someone who is immoral, cruel, and who lacks any sense of compassion. There are various synonyms that can be used to describe such individuals, including malevolent, villainous, wicked, malicious, and diabolical. Malevolent refers to individuals who have a deep-seated hatred towards others and enjoy causing harm. Villainous, on the other hand, refers to people who are unscrupulous, deceitful, and untrustworthy. Wicked individuals are regarded as evil, malevolent, or depraved, while malicious people derive pleasure from causing others pain. Diabolical is another synonym for black-hearted and revolves around the idea of evil or the devil.