She did some terrible things before she joined SHIELD. She killed a lot of people. It was to the point where Hawkeye was assigned to take her down, but he spared her.
Dude. All the b****es love when I call them "mewling quim."
No seriously, Loki fangirls are weird.
I believe the majority of that was made up for the cinematic universe. We may or may not find out more in future MCU films. Perhaps in a future Black Widow or Hawkeye movie.
But basically I think it was just because she had a lot of blood on her hands (red in her ledger) because she is an assassin and worked for the wrong people. For instance "Drakov's daughter" could refer to her hurting/killing a child, "the hospital fire" she might have started causing the deaths of innocent (even sick and injured) civilians. I believe Loki mentions (or it is implied) that Barton told him about these things.
This wasn't made up for the movie, in the books BW was brainwashed by a corrupt part of the Russian goverment to be a spy/assasin and to hate America. One of her missions was to take down Tony Stark (or something). I don't recall specific killings from that time but it is made clear in the books that she was ruthless until she overcame her brainwashing.
The extent of the brainwashing is different depending on the story. I believe that the currently accepted origin is that the Red Room trained her and implanted her with a series of false memories, such as her being a ballerina, to make her more patriotic or angry at the West. It differentiates her from the Winter Soldier and such characters in that she essentially chose her path as an assassin and willingly committed her atrocities, even if her memories stacked the deck, so to speak.
This wasn't made up for the movie, in the books BW was brainwashed by a corrupt part of the Russian goverment to be a spy/assasin and to hate America. One of her missions was to take down Tony Stark (or something). I don't recall specific killings from that time but it is made clear in the books that she was ruthless until she overcame her brainwashing.