It has been discussed as early as Avengers in 2012 that Black Widow/Natasha Romanoff has a "red" on her ledger when Loki tried to break her by unraveling her dark past and how it is dripping with blood when he brainwashed Hawkeye/ Clint Barton and made him reveal the details to it. While Nat seemed to brush it off, it was sure that recounting such a memory broke her. Finally, in Black Widow, it has been revealed.
Warning! This article may contain spoilers for Black Widow. Read at your own risk!
The Red in Black Widow's Ledger
As hard as Loki tried to break Natasha, he was unsuccessful doing it but surely his words had some hard blow on Nat's feelings when he said, "Can you? Can you wipe out that much red? Dreykov's daughter. Sao Paulo. The hospital fire. Barton told me everything. Your ledger is dripping. It's gushing red, and you think saving a man no more virtuous than yourself will change anything?"
We see Nat's face change as if Loki had tapped into something really dark from her past. It actually is dark but the scheme of Loki was not enough to break her into releasing him from his prison.
In Black Widow, it is finally revealed what the red in her ledger is and it involves a young girl that was collateral damage in his final initiation before becoming a S.H.I.E.L.D. agent. Her back story, as told in the film, was dark enough when she was forcibly taken to be a Black Widow, the same to her sister, Yelena, as they were taken to the Red Room. In the doomed mission, Natasha tries to take down the person behind the suffering she had when she was a child.
With Clint Barton, in Budapest, they blew up a five-story building where General Dreykov is and unfortunately, the daughter of Dreykov was there as well. To successfully finish the mission, or so they thought, Antonia, Dreykov's daughter was blown up as well. However, it is also revealed in the film that Dreykov escaped, and another twist, he made his daughter the Taskmaster.
As Natasha saw some hope to reconcile with the red on her ledger, we see them go toe-to-toe as Taskmaster was built to mimic the movements of her opponents making it really hard to beat her. Still, Natasha was able to do it successfully as she used the vials containing the antidote freeing Antonia from the control of her father. In the end, Antonia was even asking if Dreykov is gone as if she was thankful to finally have her free will back.