Something I have yet to see anyone talk about was the changing of the rules regarding the acquisition of the Soul Stone.
The Soul Stone “exacts a terrible price… to ensure that whomever possesses it understands its power, the Stone demands a sacrifice. In order to take the Stone, you must lose that which you love. A soul for a Soul.”
Thanos understands the sacrifice he must make. He must murder his daughter, whom he loves.
Gamora understands at last. He intends to kill her to acquire the Stone. In a wild act of defiance, she attempts to take her own life, but he prevents this from happening.
She cannot kill herself. The sacrifice must be by his action. He must murder her. And so he does.
In Endgame, Natasha and Clint travel to Vormir together to retrieve the Soul Stone, and are presented with the same challenge. They understand that one of them has to die for the Stone to be acquired. However, in this film, they play by a different set of rules. They each attempt to commit suicide such that the other one can have the Stone.
But as was established in the previous film, that’s not the deal.
To get the Stone, one of them has to kill the other. That isn’t what happens here.
Instead they struggle to save each other, and wind up with Clint clutching Natasha’s hand as she dangles over the chasm below. She begs him to let her go, but he refuses.
She tells him it’s okay, then kicks off the wall, pulling herself free of his hand and falling to her death.
That should not have worked. Clint didn’t cause her death, so he didn’t sacrifice her. She sacrificed herself. If all it took was for someone to lose and grieve someone they loved, then Gamora’s suicide should have done the trick for Thanos in Infinity War.
But Thanos understood that was not what was being asked of him. He had to kill her himself, “a soul”—his own—”for a Soul.” Clint can’t bring himself to kill Natasha, so he does not—by the Stone’s own rules—pay the price required to earn it. Her suicide should have failed the mission.