In the Flash episode "Back to Normal", why did Killer Frost try to kill Caitlin?
Frost answers this herself mere moments after being released:
Killer Frost: You know, once I saw you, I realized why Zoom didn't kill me: because I look just like you. Now he won't need me anymore.
The Flash Season 2 Episode 19: "Back to Normal"
Killer Frost believes (probably not inaccurately) that Zoom is only keeping her alive because of his feelings for her Earth-1 doppelganger, the same way a non-supervillain might keep a framed picture of "the one that got away". As long as he doesn't have the real thing, she's too valuable (from a sentimental standpoint) to kill.
But now he has the real thing, she loses her value as a memento, and her fate is likely to be the same as anyone else who betrayed Zoom. Killing Caitlin preserves her usefulness, making it more likely that Zoom will keep her alive.
protected by Rand al'Thor♦ Jan 27 '17 at 23:24
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