show has never given a real reason.
part of it is, traditionally Killer frost has been a villain. There have been multiple Killer frosts. First 2 were psychopaths. Child killing, rapist, full on bat shit crazy murderers. Among the very few irredeemable female villains.
Third Killer frost was an energy vampire. Also, 100% a villain, but more like a vampire. Like, just living without sucking up heat energy from living things is painful. She needs to kill to live. The energy given off by Firestorm, is like ambrosia.
( I may be mis-remembering which is which.)
Someone high up in DC apparently fucking LOVES Killerfrost. about 10? years ago, she was remade with Flashpoint. And started showing up in everything. Went from obscure as fuck nemesis of a z list hero, to #2 most prominent villain behind Harley quinn. More appearances then the Joker. Injustice, the moba, the CCG, every animated show, DC hero girls, Escape from arkham, young justice, etc, etc. shes been on the Justice league, suicide squad etc. The flash tv show here.
And with this reemergence, moved from full on psycho to sympathetic anti-hero. Energy vampire powers are now an ACCIDENT, caused by her evil male scientist cohorts. She resists her vampire powers, and is mostly misunderstood.
Its kindof baffling.