My working theory is that Caitlin and Frosty are both the same, but vastly different versions of the same individual. Our Caitlin represses her negative emotions fear, anger, hate, rage, etc. and her powers are closely tied to that. Killer Frost is the version of Caitlin who lives on these negative emotions, and is even borne out of them. Perhaps the Great Schism of Caitlin Frost is partly due to her past traumas, paired with anxiety about becoming the KF on earth two. Or at least you could look at the dichotomy as symbolism and see it that way.
But it’s really hard to say where Caitlin ends and Killer Frost begins without questioning things like What Makes You Be You, and Free Will, and Inherent Moral Nature (which IMO is actually a spiritual question as it deals with morality and I’m not getting into that). But we do know Caitlin does not want Killer Frost to appear (she’d rather die) and Killer Frost wants to “cure” herself of Caitlin. I imagine it’s like Caitlin is slipped some Ecstasy against her consent, causing her to loose control of herself and even though it makes her feel alive, in the moment, once she regains control of herself she’s horrified by what transpired. She would never have chosen to be Killer Frost, but while she IS Killer Frost she’s drunk on power, and then as Caitlin comes back
If you want to get “scientific” about it (use of quotation marks because 1. there’s a lot of debate in psychology about the correct theories and whether certain parts of psychology can be really be called scientific if they are not observable and repeatable, etc. 2. I’m more just using scientific theory as analogy to draw comparison, and 3. this is still just The Flash and metas also don’t exist in real life and biology also cannot explain why Cait is a human ice machine or why the speedforce is some sort of interdimensional realm ruled by pantheistic pseudo-deities, weirdly)… still if you want to, try to think of Killer Frost by looking at Freudian model of the psyche. Caitlin feels she has to be the highly moralistic "superego", while Killer Frost represents the unregulated instinctual “id” that Caitlin hides…two opposite sides warring inside Caitlin’s mind, usually regulated by the “ego” or her true self. Basically Caitlin thinks she has to be a perfect angel, and Killer Frost tries to be a perfect devil.
Now then, her outbursts are a little more extreme than say, for example, your typical case of Bipolar Disorder or other affective behavioral disorders, which leads me to believe that the unique changes in her brain when she becomes Killer Frost throws her ID into overdrive so she’s running on her negative repressed emotions. And when Barry or Julian trigger her superego, she pushes down her id to come back as Angelic Caitlin. But just maybe when Cisco gives her a choice, maybe he triggers her ego. Her true self, not perfect and angelic and also not destructive and evil. Her ability to self-regulate kicks in and allows her to reasonably choose between good and bad, right and wrong. Which of course, we know Caitlin tends to be a good person anyway. And she doesn’t necessarily need the cure with enough therapy and anger management and self-regulation. That’s basically what I’m getting, at least.
IDK??? Don’t kill me. I’m just going with what makes the most sense from what I’ve seen. TBH, I should probably also do a rewatch before writing such in depth character analysis. Also, i’m not a student of psychology at all, expect the little bit i had in school.
The Flash 4x02 “Mixed Signals” Inside Preview - Barry takes on a dangerous meta who can control technology and confronts an obstacle in his personal life: the ramifications of abandoning Iris for six months to balance the Speed Force. Meanwhile, Gypsy prepares for a hot date with Cisco, but she gets annoyed when his work keeps them apart.