There’s no other way this can end. An entire life of training to ensure the mission is always finished. Could call it a superpower. She doesn’t want this. Never would have asked to be put in this situation if she’d known the cost.
Barton, holding her hand, sees he’s beat. Shakes his head. Says, No--Don’t do this. But she has to. There’s no other way. If there were she would have seen it. A life of choices cascades before her. Missions undertaken, people helped, innocents rescued. Saved the world—more than once.
Last time they didn’t know what was at stake, not really. Not until the very end. Only when it was too late did they understand. Even then it didn’t seem possible. Only now, after years, has the amount of loss sunk in. There’s a chance to fix what Thanos did. That’s her mission--find a way to undo what he did.
Tears in her eyes, she tells him, I have to. What she doesn’t say, what she doesn’t have to say, is that she wants to. Not this, no. But complete her mission? Yes. And so she does.