The humans initially tried to deal with the Na'vi fairly, offering education, medicine, and the building of roads in exchange for the rights to mine unobtainium, but all these failed because humans failed to understand the Na'vi culture. They already had everything they needed.
Since the humans had no way to buy the rights to unobtainium, the only options they had were to A.) give up and leave - unlikely considering how much money and effort they already put into transporting all these people and machines and building structures on Pandora, or B.) deal with the Na'vi as enemies and take the unobtainium by force.