Formal/Neutral:
* Immoral: This is a general term for someone who lacks morality or ethical principles.
* Unprincipled: This describes someone who acts without regard for moral principles.
* Dishonest: This refers specifically to someone who lies or cheats.
* Unethical: This describes behavior that goes against accepted professional or societal standards.
Informal/Negative:
* Scrupulous: This means having low standards of behavior, especially regarding honesty.
* Corrupt: This suggests that someone is dishonest and willing to break the law for personal gain.
* Devious: This implies someone is cunning and manipulative, willing to use unethical means to achieve their goals.
* Depraved: This is a strong term that suggests someone is morally corrupt and evil.
* Sleazebag: This is a slang term for someone who is dishonest and untrustworthy.
The best word to use depends on the specific context and the level of formality you want to convey.