One of the most endearing aspects of the Marvel Cinematic Universe is the relationship between the sharpshooting Hawkeye (Clint Barton) and Black Widow (Natasha Romanoff). They're truly the best of friends, which is why it was so sad to see her sacrificing herself to get the Soul Stone and breaking Clint's heart in the process.
With news that the Ultimate Universe (which influenced both characters) is returning, it's worth looking at their relationship in that alternate reality. They were pretty close, just like they were on Earth-616 and in the movies, but thanks to Mark Millar, things took a horrific turn. The Ultimates 2 warped their bond in a bloody manner.
Marvel's Ultimate Universe Made Black Widow a Traitor
The Avengers in the Ultimate Universe heard rumors about a traitor among them, but no one could prove it. However, it became painfully obvious when Earth's Mightiest Heroes were destroyed from the inside and bombed. And, it was all done to help the Liberators raze America, which led to many deaths.
Black Widow was the one who betrayed them, feeding intel to a squad that raided Clint's house and attacked the others. Despite years of working together after she left the KGB and defected to S.H.I.E.L.D., Natasha had no problem with selling out her allies. The gunmen eventually killed all of Clint's family and tortured him.
Hawkeye Murdered Black Widow in Marvel's Ultimate Universe
Thankfully Hawkeye used his nails as weapons to take out his captors and tracked Natasha down. When he found her, he shot her up with his arrows, crucifying her before he killed her. This was a brutal turn that shocked fans everywhere. No one saw it coming because, up until this point, Black Widow did so much to help the Avengers, and was pretty much a sister to Hawkeye.
While a heroic Black Widow is far more appealing to audiences than a traitor who single-handedly shattered her best friend's trust, this arc did have the potential to be great. A more well-thought-out motivation for why Natasha would switch sides so quickly would have gone a long way toward making this story more palatable. However, the ambiguity readers were left with ultimately made this change of heart seem like nothing more than a gimmick.