A/N: In 6x04, when Frost gave Cisco free advice about trying to save people, it seemed like she was speaking from personal experience: "You can't save someone who doesn't want to be saved. And if you obsess over it, it could lead you down a dark path. You might make a mistake you can't fix." It made me wonder what she was thinking of, and I realized her alliance with Savitar was probably on her mind. That was Frost's first meaningful experience with Team Flash, and it was a mostly negative one. She must feel bad about that. Here's her thought process back when she discovered Savitar's identity, and chose to follow him.

At first she stayed because he was Barry. Not the Barry she knew, but the Barry he would someday become. And that was all she needed to know to trust him. If Savitar was really Barry Allen, he couldn't be bad. Just misunderstood. Confused.

He was also persuasive, and in her defiant bid for personal freedom, his known voice whispering that she could have all the freedom and power she wanted, standing by his side, was effortless to believe. The allure of finally having the unrestrained life she wanted, and being able to lead that life with a familiar face full of support, made it simple to quiet the rational part of her mind that understood how dangerous Savitar was. Because she wasn't thinking of him as Savitar. He was the future Barry. A broken, vengeful Barry, but a Barry who didn't view her powers as a curse, or think she was weak to want to use them. This Barry knew that her powers were one of the greatest things about her.

He said he needed her help for something, but in the first few days she stayed with him, he didn't elaborate. Instead he simply encouraged her to use her powers, and praised her natural ability with them. She found herself taking any excuse to use them, reveling in the feeling. At first, that was all she did. Limited thought. Plenty of fun. He smiled frequently, and even helped her practice. In those exercises, his anger sank below the surface, making him seem… like the Barry she knew. After a few days of this, his amusement seemed to fade, his cold rage taking over his expression again. She realized that rage was his most stable emotion.

She understood why when he told her his plan.

He didn't hide anything from her – including his fierce intent to kill Iris. She tried to let it roll off her shoulders that night. In the morning she still felt uneasy around him. Iris was Barry's world, and had been for as long as she'd known him. She wanted to trust that this future version of Barry wasn't the villain her old friends had painted him to be. After all, they'd painted her as a villain for wanting to use her gifts. But hearing him talk about killing Iris with such fervor felt wrong. The idea of killing Iris was wrong, and she wanted no part in it.

She could see the passion in his eyes whenever he mentioned Iris. Passion about the prospect of killing her, but also passion from simply thinking of her. It was obvious this future Barry still loved Iris, but something had twisted him, warped him enough for him to believe the only way to save himself was to kill her.

He wasn't a true villain, she decided. Not until he tried to kill Iris. Until then, he was redeemable. Anything else he did, or she did for him, could be forgiven. He could be forgiven. She could stop him from destroying two lives.

She could save him.

That was why she stayed, after she got over the rush of freedom and power and learned that this broken Barry had a dangerous plan. Everyone else had labeled him the villain. Maybe he just needed someone to remind him he was a hero.

To do that, she had to earn his trust. She had to go along with some parts of his plan, and pretend to be eager about the murder that he described as his defining moment. When he told her to kill Tracey Brand, she didn't hesitate. If she backed out now, he would just do it on his own, and she would never be close enough to save him when it really counted. So she demonstrated true effort, and allowed herself to be swept up in the display of her powers. It made it easier to focus on her objective instead of on the former friends she was fighting.

She slipped, after trying to kill Tracey. She was upset with herself for almost killing an innocent woman, realizing that not everything he did could be forgiven. Or everything she did for him. She had to convince him to stop now. So she called him Barry, and called herself Caitlin, and watched him with bated breath. Maybe hearing it from her would be enough.

His eyes darkened as he told her in a firm voice that he wasn't Barry, and she wasn't Caitlin. She nodded, and internally she began to lose hope. He was right. The familiar face that looked at her with such a distant expression really wasn't Barry. He was Savitar. He didn't want any reminder that he could be a hero. He wanted to forget everything about his past. And he was going to do everything in his power to make sure Iris died on that rapidly approaching night. He had another valid point, though. She wasn't Caitlin anymore. When she'd fought her old friends – when she'd tried to kill Tracey Brand and kidnapped Cecile – she'd acted in a way Caitlin knew was wrong. That couldn't be undone, no matter how valiant her intentions seemed. She had to be honest with herself:

She couldn't save someone that didn't want to be saved.

She also couldn't destroy him.

By the time she realized her efforts to reform this broken, dangerous Barry were wasted, she was too close. Too loyal to this man who was equally shattered and in desperate need of help and unreachable and set in his self-destructive loop. She'd stayed with him because he was Barry. Even knowing he was Savitar, knowing without a doubt that he couldn't change, she couldn't get over that glimmer of familiarity she saw in him. She couldn't run from someone with his face again.

She knew Savitar didn't fully trust anyone. He opened up to her, though. He relied on her. She couldn't save him, and she couldn't destroy him, but she could use the fragile relationship she'd formed with him to figure out his weakness. She would try to help him for as long as she could, and then do everything to help the others defeat him. As long as they were the ones to finish him. Then she could believe she'd done everything in her power to save those two lives. Iris. And the future Barry.

When she saw HR change places with Iris, she knew that was her chance. She kept silent and let Savitar think he'd won, and prepared to assist Team Flash in defeating the madman when they inevitably turned the tables. And then Savitar tried to kill Cisco. In that moment she came close to destroying him: closer than she'd thought possible.

Thoughts of Cisco and Barry stopped her. Her Barry, but also the beyond-broken Barry hiding in the blue suit. The one who now realized she'd given up on him. She stood at Cisco's side as Savitar called her weak and felt shame flood through her that she'd ever believed she could save him. She stayed by Cisco's side until the battle was over, and Savitar was dead. By Iris's hand, poetically enough. Then she snuck away, in need of time to think about her next step. She'd tried so hard to make Savitar act like he was a hero that, for a while, she'd become a villain. Her friends wouldn't soon forget that.

She would remember it for the rest of her life.