Basic Trope: A character's bisexuality underscores their villainous nature.
Straight: An evil bisexual is introduced and grosses everyone out because of their proclivities towards both genders.
Exaggerated: The villain parades around in fetish gear the entire film, nearly always with multiple men and women under their arm.
Downplayed: A bisexual character is a bit promiscuous, morbid, and abnormal, but not outright evil.
It has already been established in the work that Everyone Is Bi, so that includes the villains as well.
Alternatively, the villain is a sociopathic, hedonistic, and utterly amoral trisexual — they only care about their pleasure, and not a thing about who or what is being used, willingly or not, in the process.