I've had this São Paulo snippet floating around that I've never quite figured out how to finish, but I think it fits the bill here, so:

fire in my bones (pretty PG)

“Some fire, huh?” Clint asks, one foot propped up against the surveillance van’s black paneled doors. Above them, flames claw at the pinprick stars. “Not that I’m a pyro or anything, but man, don’t you ever just get the urge, seeing something like that?”

Natasha watches the line of Clint’s throat as he gulps from a water bottle and thinks, not for the first time and probably not for the last, that he is a fucking idiot. “You’re a moron, Barton,” she says. “Fires never end well.”

Take this mission, for example, where they’ve been flushed out of a Naples warehouse by some bottom-tier mafia lackey’s shitty idea of an extraction plan. Now the mafioso they’re here for has escaped and Coulson is scrambling to locate all the SHIELD teams on the assignment. The beautiful glow of the blaze doesn’t do much to mitigate the havoc it has wreaked.

“Oh, c’mon.” Clint nudges an elbow into her side. “Tell me you haven’t ever set something on fire.”

Natasha blinks and the inferno in front of her is replaced by one from her memories. This is São Paulo, and this is four years ago, and this is the fire she couldn’t control. She’d poured her accelerant across the research lab, careful to drench the lab assistants, careful to keep herself dry. She’d been careful, careful, careful, but that hadn’t stopped the fire from spreading. Caution, Natalia, patience, but the flames leapt from the research wing to the hospital proper, engulfed the ER and the surgery unit and--and--

“We can’t find STRIKE Beta,” Melinda May reports to Coulson in the present, tension pulling her voice urgent and tight. “They were last seen entering the warehouse via the south corridor, but there’s no response. I think we may need to call it--”

“I’ll find them,” Natasha volunteers, pushing away from the van and ignoring Clint’s sputters of protest. “Alone. I don’t need Barton.”

“I’ll never get Fury to sign off on this,” Coulson argues. “It’s far too risky.”

Natasha looks at the climbing flames. In São Paulo, she watched a hospital burn, heard the shrieking screams of dying children, and did nothing. In São Paulo, her handler clamped an approving hand on her shoulder and commended her ruthlessness, her dedication. In São Paulo, she held the matches in her shaking hands and formed her own rational thought for the first time in a decade: They are innocent. I should be saving them.

She didn’t save those innocents, but she can save these. “Just tell Fury you couldn’t stop me.” In the time it takes for Coulson to form an objection and Clint to grab uselessly at her arm, Natasha ducks, snatches up Clint’s discarded infrared goggles, and zips around the warehouse corner. The south corridor is farthest from the growing conflagration, so if she’s quick, they’ll be out before it all goes to hell.