I think she's puppet mastering these psychos she's worked with in the past, and Dex is a part of it. She's playing Dex and has all these psychos going at each other.
I think she's going to try to kill Deb to keep it all quiet.
I had the same thought, that she is using one of her patients to carry out her own serial killings so she can connect with her "greatest creation" Dexter.
Either way I am loving Charlotte Rampling in this role. she is bringing just the right mix of creepy clinical distance and pseudo-motherly affection to her scenes with Dexter.