what is wrong with nikki?
She can't seem to control or justify her inner desires, so they come out as other personalities that completely take over. Textbook multiple personality disorder. Or schizophrenia. The one where you're aware of the others. I forget which that is.
I thought the way DL died was stupid, they better retcon it by saying the company had some sort of hand in it, otherwise it seems like a pointless cheat.