Marvel Cinematic Universe Theory: Black Widow Is A Skrull
3. Her Odd Relationship With Bruce Banner/Hulk
One of the weirdest parts of Avengers: Age Of Ultron was the fact that Black Widow suddenly had the hots for Bruce Banner, despite there being nary a hint towards such a romance in the first Avengers movie.
In fact, The Avengers seemed to be setting her up for a relationship with Hawkeye, while The Winter Soldier played up the connection between her and Cap. Not that Black Widow needs a love interest at all, but Bruce Banner was the last person anyone suspected would fill such a role.
But if Natasha is secretly a Skrull, then we can look upon this relationship in a whole new light.
The Skrulls are all about the stealthy approach, and they prefer to play the long game. If there were thirty people working in a government facility, the Skrulls would take over each person one-by-one, slowly, until they had control of the entire building. Or, to put it another way, they're not going to run in guns blazing and flat-out declare war on planet Earth.
They'd surely know that Hulk would be a serious threat to them, even if they managed to take over a thousand human bodies. So why not go after him first? We don't yet know how long it takes for the movie Skrulls to fully mimic an individual, so what if it's harder depending on how powerful the individual is, and what if the Skrull needs to get a lot closer to Bruce as a result? What if Natasha's sudden liking of him is purely a way for the Skrulls to - eventually - replicate Hulk's powers?
Suddenly, a plot development that came out of nowhere starts to make a lot more sense. Why does Natasha suddenly begin to pursue the guy romantically? She barely knows him. Halfway through Age Of Ultron, she even suggests they run away together, despite the two having shared less than five onscreen conversations in two films.
Another thing that's interesting to note is that if Natasha is indeed a Skrull, then she'll have something huge in common with Banner - they both have a green-skinned monster within them. Natasha even asks him "you still think you're the only monster on the team?" a line that could be a direct clue about the Skrull lurking within her.
Having a Hulk on their side would obviously be a huge asset for the Skrulls if they need to fight the Avengers at some point, which makes it completely logical that they would aggressively pursue this particular superhero.
Or hell, maybe Skrull-Widow already took over Hawkeye and Cap before moving onto Bruce, and that's why Natasha seems to have a different love interest in each movie.