The friendship between the two girls launches the rest of the events in the novel.
Although other important events occur in their lives, this moments triggers the break in Sula and Nel's relationship, separating them for many years.
Sula's guilt is not clear-cut, since she cannot understand feelings of jealousy associated with marriage. She simply remembers how things were with Nel when they shared everything as kids.
This is the moment we've been waiting for: Nel finally vents her anger and pain and asks for an explanation from Sula.
We, along with Nel, have been waiting to find out why Sula did what she did. But she fails to give a satisfactory answer.
This would seem to be the conclusion, but Sula's death represents a winding down of the story, not an end to it. There's still important stuff to come.
This is sort of a conclusion, but an uncertain one. We learn that Nel's cries can't be contained; they have no beginning and no end, so the conclusion is open to interpretation. It could be that Nel finally finds peace by grieving for Sula, or it could be that a whole new type of pain has just been unleashed.