God Made Mud
"They may have had a hand in forming the woman she'd become, but they did not mould her in their image. Sharon wasn't sure if that was a miscalculation on their part or not." Sharon Carter aids Zemo in his bid to destroy the Avengers.
Title from Kurt Vonnegut's Cat's Cradle.
As a child, she'd modeled herself after her 'Auntie Peggy'. She dressed as similarily as one could when they spent most of their time rolling around in mud and beating the neighbourhood boys in every race they could manage to conjure up. There'd even been a time where she had wanted to dye her fluffy light hair a deep brown, as simply having a hairstyle reminiscent was no longer enough. Aunt Peggy hadn't seemed to mind it, she encouraged it only enough to keep Sharon's mother from going apocalyptic. The woman grew tired of her daughter's obsession about a week after it started, after all. The hand-to-hand combat lessons Sharon was signed up for one Christmas as a gift was almost more than her mother could take. She'd grown out of it in part by the time she reached high school, her mother had been relieved for that.
But such things like childhood fixation ultimately did have a weight as S.H.I.E.L.D. picked her up three months out of high school more than likely because of her aunt. There wasn't any glory to be had in the grunt work they started her with, but it made her drop into bed each night with a swell of pride in her chest from the accomplishments. The promotions, the missions, the mentoring that followed was everything she had dreamed off as a child.
S.H.I.E.L.D. gave her a goal, Nick Fury gave her Margaret Carter's goal, the one she learned as a child and convinced herself it was her own. At the age of twenty-eight, she stood with the option to kill a man who looked to break some of the most powerful people in the world in two. Someone who she had more reasons to kill than let live, and yet...
No matter how she spoke, how good of an agent she was, or however many time she kissed Steve Rogers she would never be Peggy Carter. She would never be anything more than a dying woman's legacy to someone like Fury; to someone like Captain America.
They may have had a hand in forming the woman she'd become, but they did not mould her in their image. Sharon wasn't sure if that was a miscalculation on their part or not.
Zemo was a man with three bodies laid parallel to each other in the trunk of his car, one considerably smaller than the others. She could not fault him for his chosen mission when she knew that fact.
Shrugging off a trained instinct was harder that one would think it to be, so she clicked on the safety on and lowered her gun with stubborn reluctance. He was good, he'd made it this far after all, but the last few steps into the very bloody heart of the Avengers wouldn't be something he could do without someone on the inside.
I've been honestly waiting for someone else to write an AU like this for years, but no one has.