Betrayal rears its ugly head throughout Sula. It characterizes many of the relationships in the story and propels much of the action. And like loyalty, betrayal appears in different ways. There is betrayal in the traditional sense, in which one person is disloyal to another; but betrayal also happens unwittingly and results from differences of opinion about some of life's greatest issues. In these cases, the betrayal isn't deliberate, but it hurts just as much.
Betrayal is vital to the story, since it creates the major conflict and propels much of the narrative.
Betrayal, while important, is secondary to the close friendship Nel and Sula share.