Miscommunication
There’s an argument to be made that Tony lost his life due to miscommunication. When he looks at Dr. Strange, he sees the Doctor holding up one finger, indicating they are in the timeline where they won. If only Tony had interpreted it as Doctor Strange intended, he might have been okay.
But no. Instead, Iron Man snatches the Infinity Stones from Thanos and uses the snap of doom to defeat the army of villains, making the ultimate sacrifice. But what if, just hear us out, Dr. Strange meant for Tony to use one of the Infinity Stones against Thanos? Nope, dying is more dramatic.
