Spoilers ahead for Avengers:Endgame, but you probably heard about this part anyway.
At the climax of Avengers: Endgame, Thanos raised hand, announces, "I am inevitable" and snaps his fingers, intending to remove half of all life in the universe and all memory of what is missing. Nothing happens. Then Tony Stark raises his hand, showing that he took the Infinity Stones while Thanos was distracted. Stark says, "I am Iron Man" and snaps his fingers, causing Thanos and his minions to go to dust. But the effort of using the Infinity Gauntlet was too much and Stark died.
Since then, fans have been saying, "But he didn't have to die, he could have just...". They are all wrong. We know they are wrong Doctor Strange told us so. He used the time stone to look at possible outcomes and the Avengers only won in a single one of them. So, sorry guys, whatever your theory is, it didn't work.
Understand just how powerful Thanos was and how skilfull a fighter he could be. In Avengers: Infinity War he beat the Hulk without drawing strength from the power stone. That's in keeping with the comics where he defeated Thor and the Thing with little effort. He was also inventive. He punched Captain Marvel in the head with no effect (showing how over-powered she is). Without missing a beat, he pulled the power stone from the Gauntlet and used it to blast her. As Thanos said, he was inevitable. Any resolution short of killing him would only be temporary and even decapitation didn't stop a younger version of Thanos from appearing to claim the Gauntlet.
"But Stark didn't have to do a full finger-snap!" Could Stark have done something less and survived? Probably not. The finger-snap represented using the Gauntlet to alter reality. It doesn't matter if you killed half of all life or just ordered a turkey sandwich, you still altered reality. When Thanos used the stones to destroy themselves it released an energy blast comparable to when he dusted half of all life, even though it was a much more localized event. Think of the Gauntlet as a genie granting three wishes.
And that ignores the cost of just putting the Gauntlet on. Every time Thanos added a stone you could see him wince. He did it again when he put the Nano Gauntlet on and when he replaced the power stone after blasting Captain Marvel. Compare that to what happened when the Hulk put the Gauntlet on. His arm was burnt up to his neck and he collapsed. And that was just from wearing it. The finger snap broke his arm. We'd been told that the Gauntlet was generating a lot of radiation, particularly in the gamma range. In the comics the Hulk practically feeds on gamma waves. In the MCU, he still felt that he'd have some sort of immunity. So trying to use the Gauntlet is death to a regular person.
"Wouldn't Stark's armor protect him from the radiation?" This is a pretty dumb question but I've still seen it. We already saw that the Hulk was horribly burnt using a Gauntlet that Stark made especially for the stones. Do you really thing that his nano-suit could improvise something more protective on-the-fly? No, Stark received a lethal dose of radiation as soon as he assembled the Gauntlet. It was hidden by his armor but you could see it on his face.
One final thought, part-way through the fight Stark asked Doctor Strange if this was the one probability where they win? Strange refused to say because telling Stark would change the outcome. That's probably because if he knew they were going to win, Stark would get over-confident and try to avoid his inevitable death. But later, moments before Stark stole the stones from Thanos, Strange held up one finger indicating that this was the one reality where they won. Stark needed that confirmation. He must have known that wearing the Gauntlet would kill him and he needed to know that his sacrifice wasn't in vain.
