A/N: This one turned out a little longer as well. Generally these will be a little shorter.
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Natasha had witnessed and dealt her share of grim shit. She'd stood by while her fellow "ballerinas" were made an example of. She'd carried out assassinations on people that she knew didn't deserve death. She had blood on her hands, red in her ledger. She could spend a lifetime trying to make up for the evil she had facilitated or directly caused. Was determined to, in fact. Of the entire little motley crew of the Avengers, she felt the most out of place. Everyone had skeletons in their closets, of course, but not all had slit someone's carotid while the wife and child slept the next room over. At least Barnes had an excuse – his brain had been scrambled by octopus-loving Nazis.
Soon after joining up with the other Avengers to keep Thor's greasy brother from enslaving humanity, Natasha had made a promise to herself. She promised that if a bullet came for one of her friends, she would throw herself in front of it if she had to. Their lives were worth more than hers. Her debt wouldn't be repaid until she placed her life on the scale, too.
She wasn't suicidal. Far from it. Suicide would relieve her of responsibility, erase the whole point. But she knew she was the most tainted, the least good. If it came down to someone dying, best it be her. Rather her than wise-yet-naïve Rodgers, so goodhearted and optimistic he sometimes gave her a toothache. Or Stark, who kept their whole enterprise afloat with his endless flow of cash and surprising aptitude for leadership. Or Banner, someone capable of so much good if he could pull his dual identities into a unified whole, if he could work with Hulk. Or the spider kid, so young and innocent to be pulled into this mess of a hero life.
Unbeknownst to the others, she had decided she would be the sacrificial lamb in a time of crisis. It was better that way.
Which was why she could only stand, stunned, as one by one her companions dissolved into clouds of dust that blew away on the wind. Barnes, T'Challa, Sam, Wanda. All while she could do jack shit to help them. There was no bullet or energy beam or toothy monster to dive in front of. No bomb that someone needed to stay behind to diffuse. Just random chance, choosing people who didn't deserve to die. Leaving her. The liar, the sneak, the murderer, the thief.
Natasha struggled to breathe. She felt like someone was sitting on her chest. She dimly recognized that she might be having a panic attack. What about Clint? Tony? Half of all life in the universe was gone. 50%. And the powers that be had chosen to leave her alive. Not good men who served their countries honorably, or the twin that had seen both her brother and her lover die. Her.
Natasha's eyes found Steve. Good, selfless, loyal Steve, kneeling near a pile of dark dust that she knew had been Bucky. He clawed at the ground uselessly, like he could mold the Winter Soldier back together. Tears welled and wet her cheeks. She knew how much Bucky had meant to him. She knew how much this entire loss meant to him. It would break him.
Natasha stumbled toward him. She grabbed his shoulder, half to keep from falling herself, and croaked, "We have to go, now. We need to contact Fury. He'll know what to do. That bastard is still alive, we can get him for this." The face Steve turned up to her was a flat wall of shock and grief. Her vision blurred briefly and rage crackled through her exhausted muscles like electricity.
If the universe had left her to carry out retribution for Thanos's atrocity, she would gladly do it. At this point, what was a little more red in her ledger?