Let's assume she's actually evil, and not working on some secret plan for the forces of good. It's relevant to ask, what kind of evil is she?

Is she evil in a Darth Vader, "yes I'm evil, but only because things didn't turn out the way I expected, and someday I will cast off my shackles and be redeemed" sort of way?

Is she evil in a Senator Palpatine/Darth Sidious, "Mwah-ha-ha I'm so bugnuts evil it doesn't actually make sense except to further the plot" sort of way?

Or is she evil in a Grand Moff Tarkin, "We have an Empire to run and little things like individual rights, and honor, and inhabited planets are either pointless or threatening" sort of way?

Valid questions I think, because the motivation for someone's corruption and subsequent turn toward evil leads to the conclusion of their story. The story I've seen leads me to believe that she started out as a Vader character - evil, but not out of choice, and with the potential for being redeemed. With Arthas dead and redemption not forthcoming, perhaps the Vader-aspect of her personality has grown into a Grand Moff Tarkin sensibility: my hopes and dreams have finally been put to rest forever, but the trains still need to run on time, there are taxes to be collected, enemies have to be fought, and new recruits have to be brought into the fold. Nothing personal, just business.

So who would run the Undercity / Forsaken faction if she were removed? It's possible that Leonid Barthalomew could turn the faction around toward a more peaceful future. Of course, it's also possible (and more likely, IMO) that Varimathras or another dreadlord could take over. I do like the idea of Leonid becoming faction leader ~ it would make for some interesting stories...