The co-creator and showrunner of The Vampire Diaries, Julie Plec, talks about the real reason the character Katherine Pierce was killed on The CW.
Looking back on the series, the show’s co-creator reflected on Katherine’s return in season 5 and admitted that bringing the antagonist back into the fold wasn’t easy.

Katherine took over Elena’s body during part of the season and Julie Plec received a cancellation from The CW regarding the character’s return. According to her, the network was concerned that Katherine appearing so heavily on the show would take a toll on Dobrev.
Read Plec’s full comments below:
“We had to negotiate for the right to let Nina [Dobrev] play Katherine again because the season 2 Katherine [story] had really taken everything out of her and the network was being very protective. [The network] and studio were being very protective and they were like, ‘You can’t do that anymore, you can’t have Katherine.’ I literally think we had to say we’ll kill her in order to get permission to use her.”
Nina Dobrev has spoken openly about how exhausting it was to play both the protagonist and one of the main antagonists in The Vampire Diaries. Although the two characters are doppelgangers, they differed significantly in their clothing, dialogue, and even movements.

Moreover, not only did they have to deal with Elena and Katherine’s differences, but they also had scenes together on occasion. All of these factors would have contributed to a more demanding filming schedule, which likely led to The CW’s concerns about overworking one of its top stars.
While all of these features were extremely rewarding for the show’s viewers, they also established a dynamic that the show couldn’t sustain. The Vampire Diaries was fully operational, thanks to Nina Dobrev’s insightful portrayal of Katherine and Elena, that helped boost the drama’s reputation.
Source: EW’s Binge podcast (via EW)