"What up forknuts?"
Bad Janet is a sentient database for The Bad Place (presumably) created by "the Makers of Light, Darkness, and Everything", much like her Good Place counterpart. Janet is able to instantly appear to anyone's side when they call her name.
Bad Janet is used primarily as a personal assistant by the Eternal Beings of The Bad Place. Most people summon her by saying 'Bad Janet'. She is able to retrieve/create any object requested for instantly. Janet also doubles as an accessible database, as she reportedly has all the knowledge of the universe.
Bad Janet by default is rude, offensive and possesses a very antisocial attitude towards others. When asked a question, she usually responds with an insult and no help at all then disappears.
From what we know about Janets in general, one purpose can be inferred: a Bad Janet can operate the train (and presumably does, to bring Shawn, Trevor, and the rest of the Bad Place crew to Neighborhood 12358W).
Another potential purpose of Bad Janets is suggested by the fact that when Good Janet glitches, strange things happen to the neighborhood around her. It is possible that Bad Janets may torment the residents of their neighborhoods by bringing about such effects deliberately.
- Even though Bad Janet is out of her own neighborhood, she can still travel between Neighborhood 12358W and her Void, though in the same episode it appears that Good Janet cannot travel between her Void and The Medium Place. This may mean that a Janet's powers are restricted to a realm (i.e., Good Place, Medium Place, Bad Place), rather than a specific neighborhood. If so, it is a clue that Neighborhood 12358W is part of The Bad Place, and also that The Medium Place really is separate from it.