I love how nicely this movie goes back to 2008.... (SPOILERS)
Spoilers and spoils of war ahead. Also Thanos' head, sitting on the ground.
In 2008, Iron Man and The Incredible Hulk started the official MCU series (and yes I still prefer Hulk's movie out of the two, I hated that it got overshadowed).
The end of Hulk had Banner first learning to control Hulk through meditation.
Iron Man had just turned against weapons manufacturing.... By then creating an even more dangerous weapon in the IM suit.
To pay a tribute to these two who began this franchise, both Hulk and Tony get to snap the gauntlet.
They have also each finished their arcs... Banner and Hulk are one, there is no delineation.
As for Tony, he has pretty much given up his weapons in this movie... And he also gets yet another step up to a far more dangerous one.
But using the gauntlet to peacefully remove Thanos and his army is very contrary to Tony's Iron Man crusade... But far closer to the ideal that he developed in that first movie, wanting to do away with weapons manufacturing.
And Thanos' greatest mistake... Using Tony's gauntlet. He hadn't had any experience with Iron Man yet, not for years... so Thanos was not prepared for Tony's tech, not prepared for Tony always having a backup protocol and a long line of "just in case" fallbacks.