If the theory that Anthony Fuchs of We Got This Covered has mentioned has any merit then it could explain quite a bit about Natasha’s desperate dive into nothing on Vormir during Endgame, and it could very well give us the motivation that made such a sacrifice possible. There was a two-year gap between Captain America: Civil War and Avengers: Infinity War, a period of time which saw Natasha return to her homeland apparently and reunite with her parents and sister. Eventually though, to keep the continuity of the MCU the way it is, she would have to part ways with her family, which might make a lot of people wonder why. It would likely be obvious to a lot of people that she would feel obligated or would want to return to the Avengers for one reason or another, but at the same time it’s still kind of difficult to think that she might just up and leave a family she just reconnected with. But hey…reasons. In any case, Natasha would come back to the fray with her companions and seek to stop the mad titan, Thanos, from destroying everything, only to fail as was seen in the movie. That however is where this theory comes into play. It has nothing to do with Thanos somehow being connected to Natasha’s life in an intimate way so get that out of your head. But seeing as Thanos is responsible for so much loss in the Infinity saga it stands to reason that Natasha might have lost just as much as anyone.
What if Natasha’s family was turned to dust by the Snap? It certainly has the distinction of being plausible enough to work, especially since it’s something that could be easily added in if the powers that be decide that it’s a necessity. Thomas Bacon of Screenrant tends to think it might be possible. Think of it this way, they’re alive and well when she leaves them, but upon the end of Infinity War when the tally of those lost to the Snap is being added up, it’s very possible that Natasha could find a way to check in on the family and see if they’re already after watching so many people just fade away into nothing. Upon finding that they too had disappeared it’s also likely that her grief would be amplified even as she tried to quell it in any way she could. Given her highly emotional state in Endgame, which is easily understandable since nearly everyone was feeling it in their own way, it would make sense that she would look like she was falling to pieces.
Black Widow is a tough character, she was trained to be that way and she was taught to be emotionless when she needed to be and entirely companionable at other times. Depending on the version that people have been shown, as in the comics vs. the movies, she’s been a very cool customer in some situations and an absolute nightmare to her enemies in others. But in the MCU she’s been shown to be all business at times but also capable of breaking down in a very human manner that shows that she too has limits. The deaths of her family might be just enough to push her over the edge and make her the unstable mess that she appears to be in Endgame, at least for a while, but it would also put that extra steel in her spine and make her decision on Vormir appear to be a lot more sensible since it would be the admission that she didn’t have as much to go back to. The only argument that I can think of at this point is the idea that once all the stones were together she had to think that the people taken by the Snap could have been brought back. Of course given that she was with Clint that argument is destroyed again since despite wanting to see her family once more she would also reckon that Clint’s family might take precedence as he had young children that needed their father, whereas her family had a host of issues and history with each other that wasn’t all positive despite the idea that she’d reconnected with them. If this theory holds up then Vormir was a huge judgment call on Natasha’s part, and one that she didn’t have a lot of time to think about.
This is the only problem with making origin movies after the main part of the narrative has been given, there’s so much to connect that it’s kind of like trying to build a house from the inside out. It’s possible, but everything has to connect in a way that makes sense and will work with everything else that comes along. There’s no doubt that the movie will be impressive, but hopefully it will fit in a way that makes it all click.