This person is insane and probably murderous — and they’re bisexual, the icing on the depravity cake! TVTropes.org points out
This is a very different phenomenon from the Psycho Lesbian trope. Whereas the Psycho Lesbian is usually violent or deranged out of unrequited love and/or jealousy, the typical Depraved Bisexual is bi because, well, why not? Their willingness to sleep with everyone they can is just one facet of their being Ax-Crazy.
And if they’re not murderous, then they’re still manipulative, mean, and vindictive.
Why this can be bad: This is essentially always bad, mostly because of what TVTropes.org says. Many writers make their depraved killers bisexual only because they want to show just how “bad” their villain is, and all that does is present bisexuality as something hypersexual and evil. Bisexuals already face their own brand of oppression, and they definitely don’t need you to add more content like this to their lives. The big reason this character is so detrimental to the representation of bisexuality is not only the reason stated above, but also that this character is often the only bisexual presented to the audience, therefore linking the sexuality (bisexual) and the morality (evil) together. On top of this, writers like to think that bisexuals will have sex with anyone, when this isn’t true at all. If you’re heterosexual, will you have sex with anyone of the other sex? Probably not. Bisexuals have standards, likes, dislikes, turn-ons, turn-offs. They’re not insatiable nymphomaniacs. They’re just people.
How you can fix it: Think really hard about why your crazed villain needs to be bisexual. If it’s like the above (to show how depraved they are), just stop. If you continue to insist that this is “just who they are,” then you need to work on making sure they aren’t the only obvious bisexual in the work, otherwise they’ll become the token stand-in and representative of the entire sexuality.
Bottom Line: Depraved bisexuals sure as hell better not be the only bisexuals in your story, nor should they be bisexual because they’re depraved. And remember: bisexual does not mean “will sleep with anyone.”