Sinful, villainous, criminal, or wicked, especially when noteworthy or notorious for such characteristics.
“Vikings, pirates, outlaws, Native Americans, Confederate soldiers, Roman soldiers, hessians, and German soldiers in both world wars all committed many nefarious deeds.”
“Aliens have a nefarious connotation in many science fiction books.”
Of, relating to, or appropriate to a villain.
Wicked, offensive, or reprehensible in nature or behaviour; nefarious.