As a character who flits from doing good to indiscriminately murdering at the drop of a hat, it's easy to see how Black Widow could have done a huge amount of terrible, terrible things in her time in comics, especially since she first appeared in the Ultimate Marvel Team-Up #14 as a supervillain.
While the vast majority of this can be put down to the brainwashing of her former KGB trainers and captors, not all of Natasha's worst actions take place in her days as an enemy of the Avengers. Indeed, Black Widow has been responsible for a whole host of awful antics while entirely free of the chains of her old life.
Unfortunately, many of Nat's worst moments occur when she's arguably trying to do good things - as a lifetime of being a hardcore assassin has left her willing to do literally anything to get the end result she wants, which leaves her slaughtering and maiming completely innocent folks for *Hot Fuzz voice* "the greater good".
With a rap sheet that includes fratricide, cannibalism, and almost ending the world, it's kind of a surprise that Natasha Romanova is still considered a hero in the first place.