I agree, Brierrose. I don't think that the third World Killer is anyone that Kara knows and I don't think that it is Kara either. When they put the four photos of the possible WK on the screen at the end of
For Good, they stated something about two of them being the WK's and Julia was one of them. So, that leaves the remaining 3 as possibilities for the last one.
I can't remember if we talked about this or not but, going back to what Coville stated in
Reign, he mentioned the End of Days. This could be somewhat of a parallel to the Apocalypse that is discussed in Christianity's Book of Revelations from the New Testament. Coville talks about signs, which are also referenced in the Revelations. In the Apocalypse, there are four horsemen (seals that are opened) with three of them representing war, famine and death. I think that Reign, Purity and Pestilence are representing three of those symbols but, are modified to fit the SG mythology. If so, then maybe a twist at the end of the season is that there is a fourth WK and depending on one's interpretation of the Horsemen, this last figure could either be someone who represents righteousness or evil. If it is righteousness, then perhaps that could be Kara. If it is evil, then maybe it is one of the scientists (possibly even Alura or Zor-El) or the hologram in the Fortress of Sanctuary.
In the Book of Revelations, after these four seals are opened, the remaining seals are opened destroying the world by earthquakes and burning - which could be how the world's in the 31st century end up being destroyed. And maybe it's not the Legion's DNA that is necessary to stop the WK but, Kara's and she doesn't survive in the original timeline because instead of eliminating Sam, Julia and the third one, Kara tries to save them and it backfires. Perhaps Imra and Brainy know this and if Kara is indeed involved in some way, then that is why they came back and didn't tell Mon-El.
But, on the other hand, when Kara was interrogating Purity, Purity mentioned something about her and the other gods eating Alex's heart? That almost sounds more like Greek mythology to me so, who knows, maybe they're combining some aspects of Christianity and Greek mythology
![;)](/media/com_easydiscuss/images/markitup/emoticon-wink.png)
.