Vogel and Dexter become targets of a serial killer who initially believes Vogel is an ex-patient. They started to train some random person they initially suspected was the killer, but he was murdered too. 

As the show neared its end, Dexter found out the killer is actually his murdered neighbor’s boyfriend and Vogel’s son, Oliver Saxon, who had faked his death in a fire. Vogel initially tried to protect Oliver because she naively thought she could change him, but realized that he’s a lost cause and gave Dexter the go ahead to slice him to bits. 

But it was too late. Dexter and Oliver both slit Vogel’s throats in front of Vogel, removing the single parental figure Deb had left. 

As all of this was going on, Dexter was planning a life in Argentina with Hannah and Harrison. The only thing holding him back is his desire for vengeance. Oliver nearly satisfied his need for revenge when he wrapped Oliver around a table and realized that bloodlust is not something he feels anymore.