This first half we have seen villians that have been deadly, but like with most of the villians in Teen Wolf they had a tragic backstory. One argument I have heard is which one of the two deserved redemption the most or should've died all together.
Deucalion[]
His form as a werewolf says it all; a Demon Wolf who is not afraid to kill anyone who gets in his way. Understand now he was a pacifist with a vision of peace between the wolves and the Hunters. Gerard (geriatric pyschopath) kills some Hunters, blinds Deucalion, and frames him for the crime. After being blinded, his Beta, Marco, decided he was unfit to be an Alpha and decided to take it from him. Deuc viciously kills him in retaliation and discover he can see as a werewolf and by killing his own Beta he absorbs their power. He then forces fellow Alphas Ennis, Kali, and the Twins to do the same as a way to form the perfect pack.
Now, he forms it because he wants to not be threatened by any like the Argents or Druids alike. Adding a True Alpha like Scott is icing on the cake, but he is also threatened by his poweress which causes mental conflict.
Jennifer Blake[]
Jennifer, aka, Julia Baccari was a druid emissary and best friend to Kali. She felt betrayed when Deucalion made Kali not only kill her own pack but her as well. Saved by the Nemeton, she vowed revenge on the Alpha Pack. Her face mangled and scarred, she had to use alot of magic to look normal and it constantly reminded her of Kali's betrayal. Jennifer becomes a Darach and starts to make sacrifices to the Nemeton to recieve enough power to annililate the pack.
She seduces Derek on to her side as a way of not killing the final three sacrifices. This shows she doesn't wish to kill these people but feels it is the only way. She eventually kills Kali and incapacitates the twins.
The outcome[]
Jennifer was too filled with revenge and hate that not even Derek's pleas to stop would make her change her mind. She was ready to kill Derek because he did not believe her cause. It would be safe to assume she was beyond hope. She still clings to her dark ways and goes to the Nemeton to be healed again only to be stopped by Peter Hale.
Deucalion's sight restored, he sees the result of his plans on Jennifer. After being amazed by Scott and his newfound Alpha status, Deuc decides to save Scott's yellow eyes by slicing Jennifer's throat knowing that his eyes are already blue if he wasn't an Alpha.
After weighting it out, I think Jeff made a good decision on giving redemption to Deucalion instead of Jennifer. Despite his crimes he had intentions that didn't seem to lead to "simple murder" other than just protecting his own kind. Maybe he will be the man of vision Talia Hale once called him again.