Fans of the MCU know that, in Marvel Movie canon, Black Widow died on the planet Vormir, sacrificing herself in Avengers: Endgame so the Avengers could acquire the elusive Soul Stone. However, in the comics, Natasha Romanoff died right at home on Earth, and it was one of her most stalwart allies who did the deed.
The event took place during Marvel's crossover event Secret Empire. In an America taken over by Hydra, Black Widow was one of the last active Avengers, taking Marvel's teen heroes under her wing and trying to teach them the deadly skills they needed to survive as insurgents in a world turned upside down. Sadly, it was her commitment to protecting the heroes of tomorrow that got her killed by the last hero you'd expect.
Whether Captain America was in his right mind when he killed Black Widow really depends on who you ask. Nick Spencer's run on Captain America led up to Secret Empire, revealing that Steve Rogers had been compromized and now served (and wished to lead) the fascist organization Hydra. What muddies the waters is the use of the reality-rewriting Cosmic Cube. In Spencer's stories, it's suggested that the Marvel reality fans know and love is actually a fake - Captain America was always a Hydra sleeper agent, but in the waning days of WWII, the Allies got hold of a Cosmic Cube and (along with other changes) turned him into America's true blue hero. The evil Captain America is therefore the original, with his evil intentions merely suppressed for most of Marvel's comic history.
This was a hugely controversial detail at the time, and while Spencer's story ended with the resurrection of a more familiar Captain America who represented a pure distillation of the American Dream, stories since have treated the idea of an "original" fascist Cap as a lie - something Hydra created with their own use of the Cosmic Cube rather than Marvel's original reality. Whatever the truth, Secret Empire saw Hydra Cap seize control of the United States, and Black Widow resist him with all her might.
Tragically, while Black Widow's death was heroic, she wasn't even Captain America's intended target. Forewarned that he would be killed by Miles Morales, aka Spider-Man, the evil Steve Rogers aimed a killing blow at the teen hero when they met in battle. Black Widow, who had been hoping to spare Miles by killing Steve herself, was just in time to push the web-slinger out of the way, which sadly meant she took a blow from the Captain's angular shield directly to her neck, killing her instantly.
Of course, Natasha is still running around today, so what happened? Well, 2017's Tales of Suspense revealed that Natasha was somehow "back," prompting Hawkeye and the Winter Soldier to investigate. Ultimately, they discovered the new Black Widow was actually a clone of Natasha, into which all of the original Black Widow's memories had been transferred. Whether or not this plot twist could be used to bring back Scarlett Johansson's version of the character in the MCU is up for debate, but it remains a pivotal moment in the character's history, giving the Marvel Universe a returned Black Widow while ensuring the death of the original Natasha Romanoff stays squarely on the shoulders of the evil (and perhaps original) Captain America.