"Something about needing her more than killing… I'm afraid we're heading for a 'true love conquers psychopathy' here."
The Natural Born Killers ending?

I originally thought Vogel was the brain surgeon, but I remember an earlier episode showed her discovering the brains and freaking out. It would be different if that was a cover while Dexter was there, but she was alone.

It should have been explained in VO.