The Infinity Stones deliberately killed Tony
I don't think this is true, but it's a really interesting theory. The idea is that being brought all together gave the Infinity Stones a kind of sentience. They were innocent and unknowing when Thanos did his Snap and in shock when they were destroyed (or allowed themselves to be destroyed?). When the second set of Infinity Stones are brought together, they have an awareness of their power and the damage that's been done. So they bring back all of the dusted while doing as little harm as possible to the Hulk, because they can see his past and future and know he never intended harm. But when they get a look at Tony's past and future...
***
They were...Aware. A sort of collected consciousness along with a collective conscience. They Felt the one called Hulk, Bruce, Doctor, Professor, Monster, Hero. They Felt him, Knew his past, Saw his possible futures. Knew he was reaching so desperately to bring back those that They--the Stones, the Other Stones, now gone to less than dust Themselves--had taken from this universe. Knew in his past this Bruce wished to do right but did wrong. Knew he admitted his mistakes and made amends. Knew he would go forward wishing only to help and be left in peace.
He was Worthy. Their Powers reached and transformed dust back into beings, restored Life to Nothingness. But even She--the one called Natasha, Natalie, Romanova, Rushman, Agent, Widow, Assassin, Spy, Traitor, Heroine--was beyond Their reach. All They could do was keep Their power from consuming the one who directed their energies. He would be forever Changed, but They would let him Live.
Then chaos reigned. As They bounced and traveled underground, They stretched Their awareness. Another Thanos, Madman, Destroyer arrived. Like the one who had made Them--the Stones, the Other Stones--cause so much destruction, the other Thanos filled the ground and air with beings obedient to his will.
Only one hero stood against Thanos--the other Thanos. They could not Know this hero--not from beneath the ground--but They could Sense this hero was Worthy as he, wounded and so tired, struggled to his feet and stood alone against the full might of Thanos--the other Thanos.
Then They passed from hand to hand--Sensed each hero and heroine as They waited for one of these Worthy ones to call upon Them. Then Thanos--the other Thanos--held Them once more. They Felt him, Knew his past and the future he wanted. A future where all this universe turned to dust to be rebuilt to his vision. He was NOT Worthy.
Before They could destroy Thanos--the other Thanos, They were lifted from their resting places and set in a new gauntlet. Held by Iron Man, Stark, Tony, Husband, Daddy, Merchant of Death, Billionaire, Genius, Playboy, Philanthropist, one-time Hero. They Felt him. Knew his past. Knew in his past Tony had done wrong and not cared for the damage caused. Unless forced, had not admitted his mistakes, apologized, or made amends. Had gained when others lost, and never gave back enough to balance. They Saw his possible futures. A willingness to let worlds suffer so long as his life was unchanged. A weapon of death made in secret floating above, missiles pointed at anyone Tony wished dead, whenever he willed it.
They were ready as Iron Man called upon Them to destroy the villains in this battle who would cause harm to this universe. They Knew Iron Man was NOT Worthy. They did not shield him from Their energies as They performed this much smaller task.
When They were finished, no villain was left alive. Not even Iron Man.
***
The one who stood alone against Thanos--Steve, Punk, Rogers, Cap, Hero--carried them home. They did not Know him for he did not call upon Their power. But They Sensed him as he carried each of Them to Their home. He was Worthy.
They were content.