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Scoundrel
Negative Connotation
Scoundrel is always negative, suggesting that the person is not just bad, but also tricky or deceitful. 
Historical Use
In historical or classic literature, 'scoundrel' is often used to describe deceitful men with no honor. 
Informal Speech
While it's not vulgar, 'scoundrel' is informal and might not be suitable in a professional context. 