Revenge Season 4 spoilers: Why didn't Emily kill Victoria?

ABC's Revenge Season 3 finally ended its last chapter, and though not everyone is content with how the season ended, all eyes are now focused on Revenge Season 4 spoilers ahead.
Emily has finally avenged her father, clearing his name and taking down Conrad. Aside from Conrad's death, the season concluded with Aiden's murder, the return of David Clarke and Victoria screaming in a mental center.
As to why Emily did not kill Victoria, the former wants her to live – but live miserably. After all, suffering in this life is a sweeter revenge than merely dying. With Season 4 nearing, Victoria will feel the revenge and anger brought on by her wrongdoings, as everyone should realize man contributes to his own unhappiness.
As the fourth season unveils, Revenge followers wonder who will be the next interesting villain, or about the additional member of the antagonistic group.
Will the season feature the return of David Clarke, who made viewers wonder why he was so hesitant on being with his daughter? Maybe he is not innocent after all; and why did he keep it clandestine not letting anyone know he is still alive?
If it is still Victoria, then she has been keeping that position from the beginning. To be a villain in Season 4, she should have her way to off the prison and can become even more threatening.
Well, it can be Conrad – that is, if he is still alive. If he lives, he will eventually meet David and viewers will get to see how things will turn out.
Even the heroin Emily can turn into a villain.
Lastly, someone entirely new can come in and become the next baddie.