RIP Caitlin Snow. Everybody's favorite doctor on The Flash flatlined after being attacked by a magical villain in "Abra Kadabra," and that's where her troubles began. After Cisco, Julian, and H.R. failed to revive her, Julian ripped off her power-blocking necklace, letting her Killer Frost powers take over to heal her. Caitlin died on The Flash, and came back as Killer Frost.
Before Caitlin's heart stopped, she was actually proving herself to be the strongest of them all. When the baddie of the week, Abra Kadabra, caused an explosion that left Caitlin with shrapnel in her side, she stayed awake so that she could talk Julian through performing her own surgery. And she looked like she was recovering well, until she passed out while talking to Cisco. Despite Cisco, Julian, and H.R.'s best efforts to revive her, it looked like Caitlin was dead. Until Julian ripped off her necklace, despite the fact that Caitlin had told them that she would rather die than become Killer Frost. After she healed herself, Caitlin's body let off a wave of cold energy and stood up a new woman. Her hair was white, her eyes icy blue, and she shot ice out of her hand. She didn't look like Caitlin, she looked like Killer Frost, and she acted like it too.
Caitlin's transformation into Killer Frost has been a longtime coming — it was one of the things teased in the future Barry is trying to prevent, but it's clear that Team Flash was not ready for this. Putting aside the fact that Caitlin's transformation into Killer Frost brings Iris one step closer to being murdered by Savitar, the arrival of Killer Frost also presents a bunch of new problems. What if Cisco and Julian and Barry can't bring Caitlin back this time? Furthermore, as this episode made very clear when Caitlin had to stay awake to talk Julian through performing surgery on her, Team Flash needs a doctor, and so far they don't have anyone capable of replacing Caitlin Snow.
It's unclear whether or not Team Flash will be able to defeat Killer Frost and save Caitlin Snow this time. She might be too far gone. One thing is clear: the Caitlin we know and love will never be the same.