Why did Tony Stark have to do the snap?

Question

In the movie scene, it seems to me that at the point when Tony snatches the stones off of Thanos' gauntlet, the situation was pretty much in control already. With Captain Marvel having destroyed Thanos' flagship, the Terrans seemed to get the upper hand on their own, and still Tony chose to sacrifice himself.

Why couldn't he just run off with the stones for the moment? Defeat Thanos' army conventionally, or just hand off the Gauntlet to The Hulk who can actually survive? I get that his sacrifice might have saved a (small?) part of the Terran army, but apart from that, given his pretty strong will to survive, was there an immediate reason for him to go for the snap right then and there?

Answer 1

Why risk it? Are you forgetting what had just happened before Tony’s Snap? The Avengers had just had a desperate fight with Thanos over the Gauntlet in which Thanos won, Tony just sneakily beat him. Thanos has the upper hand at that point. Why risk things for longer? It was do or die at that moment as Thanos has just shrugged all of them off.

It was the winning outcome as well. Strange had just raised a finger to Tony to indicate this, why wait on a winning outcome if you're about to win. You don’t take the football to the goal line and then wait before scoring for no reason. You certainly don’t leave a dangerous situation, dangerous just because you think there’s a chance for things to work out better.

And of course Tony doesn’t know he is going to die by performing the Snap. He’s seen three snaps happen with the Gauntlet and his is relatively minor in comparison. For all he knew he was going to be heavily injured but survive it.

Answer 2

Tony knows he is going to die, and also knows that is the only way to end the game. ;)

He understands this when he looks at Doctor Strange.

He realises that this is the only one possible winning scenario Doctor Strange saw when he travelled to the future 4 million times.

And he realises that Doctor Strange gave up the time stone to save his life because Tony Stark was the only one who could do the snap by sacrificing himself (when he said: "we are now in the end game").

Also, when Tony Stark checks in with Doctor Strange during Endgame, asking if this was it, if they were in the reality where everyone survives, Doctor Strange's cryptic reply -- "If I tell you, it won’t happen" -- is a big hint to Tony's role. Tony would have to sacrifice his life, to miss out on his daughter's future. But after meeting up with his father in the past, it seems like he knows he wants to be the kind of man who put others before himself.

And so he does it, he sacrifices himself: He knows what will happen and he is at peace with it.

Evidence for this:

1. This is supported by the writers Christopher Markus and Stephen McFeely in this interview. In their own words: “The only way this works is if Tony dies. If you go back and watch Infinity War, when [Strange] says ‘one,’ Benedict … is just choked on emotion. Now that you know the end, go back and watch that moment. He means, ‘You’re going to have to die, Tony.'”

2. Even the director Joe Russo says in this interview: "And you know, I think again, as a father, it’s very difficult to watch a character like that have to make a decision, knowing that he has to give up his life at the cost of his daughter losing a father and his wife losing a husband."

Answer 3

Scott Lang, in his official (in-universe) autobiography Look out for the Little Guy! gives us an overview of the battle. In short, Thanos' troops are everywhere, the fight is not going well for the heroes and Thanos has already come within a whisker of absolute victory.

At that moment, things suddenly started going really sideways. Thanos’s mob was closing in, we were out of options, and even cavalry after cavalry of new heroic team-ups weren’t turning the tide.

We’d tried everything. We’d fought like hell, to hell and back. We’d worked together, we’d worked individually, and we’d kicked so much bad-guy butt our feet would never heal from the bruises. Finally, I felt the weight of that soul-sinking sigh that Cap had given me back at Avengers Compound when telling me about the Battle of Wakanda.

It seemed like it was all over as Thanos came to claim the gauntlet, as Thor, Cap, Captain Marvel, and Tony each fought heroically to take it back from him. But Thanos beat them all back, finally flinging Tony to the ground with a proclamation that seemed to sum up the vibe of the moment: “I am inevitable.”

Tony has the stones for a fleeting moment and isn't going to risk them falling back into Thanos' clutches. He managed to get the stones using guile, but it's not something they can count on happening again.

And that’s when Tony held up his Iron Man–suited hand, now glowing with Infinity Stones. He had used that tussle to pickpocket them all more stealthily than anyone I met at San Quentin could have dreamed of.