... Harmonica despises Frank more than anyone in the world, Frank traumatized him when he killed his brother, he is walking around for all those years with the trauma, symbolized by the harmonica that he wears like a cross around his neck, (trauma which isn't relieved until he kills Frank, at which point he can finally tear the harmonica off his neck)....
Yes, this is the best explanation to keep the source of Harmonica's motivation within the frame of this opera.
I do not want to spoil this drama, but it comes to me an alternative. Its not inevitable that he has been carrying his cros all the time; he might as well been waiting for the right time and place to deliver the justice. His hate towards Frank might have been transformed to a postponed impersonal decision to exercise the justice in a fair way based on law. He comes to act as a Court, if you wish as the Supreme Court.
Although the legal institutions
in them days might not be perfect, still there were rules and law punishing a criminal act (
It was all in order. Seals, signatures, everything). And Harmonica does not want to be a criminal. Officially he has no evidence against Frank since
That was always one of Frank's tricks. Faking evidence. In fact the duel is initiated explicitly by Frank, only implicitly by Harmonica. Thus killing him in the duel from the legal point is a selfdefence for him and the revenge on his brother.
I this respect, the impressive sound of harmonica would not represent the deep long lasting Harmonica's trauma, but a robust and unanmbiguous announcement that the Judgement Day has come.
What contradicts the above alternative is the long row of
more dead men starting with Dave Jenkins, which had been growing probably continually through some time and Harmonica had been following it with a rising tension that reirritated his old aching wounds; another moment breaking my alternative is the "IF situation" if Frank had come to the Cattle Corner to make the appointment and Harmonica killed him - what the hell of legal thoughts came through Harmonica's mind to keep him clean!
Anayway, as I perceive it, Harmonica acts as a judge without emotions bringing the justice, while Frank acts as a villain wanting to kill Harmonica since he spoiled
something that belongs to him and posing himself as a MAN is his decision to act as a MAN only in the decisive moment of his existence.