You don't expect that opressed minority character to be your first villain, right? But that's what happens in E.R.
Even having a villainous character in the show is pretty strange. E.R. is usualy a melancholic series, where success and especialy happiness has a rare appearance, funny moment constantly get overshadowed by other events, or sudden point of view.
What happens here perfectly fits the title, but whil the arrest of Carter is a funny side-story, the main event is actualy Jeanie Boulet's story, a black, HIV-diagnosed woman.
As the story goes is, the hospital is under financial pressure, so they have to release certain employees, one of the becoming Jeanie. Sure, she is HIV-positive, but the named factor is financial: employees with less time at the house need to be payed less, so bye-bye Jeanie. Until this, it is your usual melancholic ER-episode.
Also, to sweaten the deal, the Boulet-couple is offered work in another city. So Jeanie has every reason to move on with her life: she is not wanted here (though she is offered to be taken back later, when finances stabilized), she just found happiness of househuld at the side of her partner, currently ex-husbnad of hers, and she has at least the same deal in Atlanta, where she doesn't even have to get into situations endangering patiences.
This is when Jeanie Boulet decides to stay. She throws away the job-opportunity, also her new-found happiness of love, AND along this the respect of her colleagues in Chicago.
She stays to feel powerful. With other words: she stays, because she is a villain.