I don't think I've ever seen a movie that has been defended as much or felt the need to be explained in all my life, especially with the film only being released a week, but The Avengers: Endgame takes the cake. Following Captain America, now the Russos defend the decision to kill off Black Widow and the controversy that has ensued.
Prior to diving into what the Russos have to say, The Avengers: Endgame saw Black Widow sacrifice herself for the Soul Stone instead of Hawkeye. The two battled it out, but Black Widow was the one who pushed off and killed herself to save the universe.
Regarding the controversy, apparently, there has been a question amongst feminists (seriously) regarding why Black Widow was killed off and that it should have been Hawkeye. The argument in favor of saving Hawkeye is because he has a family, while Black Widow doesn't have a family and is unable to have children because she was forcibly sterilized as part of her Red Room indoctrination as a Russian assassin. So the feminists aren't happy that Black Widow was killed off because she can't have any kids and is a female, while Clint is a male.
Speaking with EW.com the Russos offer a few reasons why Black Widow is the one to die, with a reason including simply that she is a better fighter, so she wins against Hawkeye.
“It’s a fight to see who’s going to kill themselves,” said Joe Russo. “It’s a crazy concept for a scene. And as you’ve seen in The Avengers, she’s a better fighter than he is. So when it comes down to a fight between the two of them, she wins.”
Joe Russo also says that Black Widow is definitely not throwing her life away.
“The theme of the movie is, can you change your destiny, and what does it cost to do it? And are you willing to pay that cost?” he said. “It’s a resounding yes from the Avengers. In [Infinity War] they said, ‘We don’t trade lives,’ and there was a desire to protect. And in this movie, there’s now a desire to sacrifice in order to accomplish the goal."
The article also confirms that in order to get the Soul Stone, something you love has to be sacrificed and that it is obvious Black Widow and Hawkeye love each other.
After all that they’ve been through together — most recently her rescuing him from his bloodthirsty path as the vigilante Ronin — there’s little doubt that Natasha Romanoff and Clint Barton love each other.
In an interview with the writers on The Avengers: Endgame with the NYTimes that discusses alternate and deleted scenes, it is also revealed that visual effects producer Jen Underdahl wanted to keep Black Widow's death in the movie as she felt Widow made a worthy sacrifice: “Don’t you take this away from her.”
The same interview also explains why Natasha didn't have a funeral scene the way Tony Stark did at the end of The Avengers: Endgame:
MARKUS Tony gets a funeral. Natasha doesn’t. That’s partly because Tony’s this massive public figure and she’s been a cipher the whole time. It wasn’t necessarily honest to the character to give her a funeral.
It is also revealed why they didn't bring back Black Widow through use of the Infinity Gauntlet (Banner also said he tried but couldn't, I believe):
MARKUS The biggest question about it is what Thor raises there on the dock. “We have the Infinity Stones. Why don’t we just bring her back?”
McFEELY But that’s the everlasting exchange. You bring her back, you lose the stone.
In a Q&A in China, Joe Russo also touched upon attempting to bring back somebody who sacrificed themselves for the Soul Stone, noting Widow is gone forever:
No, the process is irreversible. Even if you have returned it to its original location, you wouldn't be able to get the person back. In fact, it's not really returning the stone, more like putting it back properly. The tribute soul for the Soul Stone will forever be sealed in that place, therefore Black Widow is gone forever.
Joe Russo also touched upon why no funeral for Black Widow:
Did you forget when the heroes were mourning for her after when they returned from the past? Maybe her funeral happened off screen. Maybe it will be shown in a future installment --because there are still tons of stories in MCU that are waiting to be told.
While Black Widow may be indeed gone, she is not gone for good as Marvel Studios is developing a Black Widow movie starring and produced by Scarlett Johansson. The question remains, however, how the movie will fit into The Avengers: Endgame: Will it be a prequel as has been reported? Or now that The Avengers: Endgame introduced alternate timelines, will Black Widow's story continue within the multiverse?