Natasha Romanov is by far the most mysterious out of all the Avengers - and she has good reason to be. Her past is shrouded by both darkness and blood, and this is not just a cute little metaphor. With a name 'Black Widow' we might automatically associate her with evil, and it is true that she doesn't have the cleanest morales of the Avengers. However, she has changed in the time period we know very little about before the Avengers or even Iron Man 2, and it is easy to see how and why.
We know that Natasha regrets her past, and blames herself for a considerable amount of it - hence why she refers to all the blood she has spilled in her time