(Beware: Spoilers ahead.)
While Furiosa was aware that Dementus had lost his family, she sees the rawest side of him yet.
In their interaction, he explains how killing him would be more merciful than letting him live.

Furiosa / Warner Bros.
He doesnt fear death or vengeance.
Dementus paints a bleak future and how this cycle of never-ending agony only continues.
Inadvertently, though, he shows Furiosa what can happen if she doesnt find peace within.

Warner Bros.
Case in point: The War Rig driver, Praetorian Jack (Tom Burke).
Jack harbors no ulterior motive besides helping her here.
And Furiosa does this by freeing Joes wives and escaping with them from the Citadel.

Warner Bros.
Metaphorically, theres a lot to unpack here.
First off, the seed serves as a physical symbol of Furiosas hope.
At the same time, she still falters and doubts herself, often needing a reminder of her promise.
In the case of Furiosa, she no longer needs the physical seed to remind her of hope.