É uma boa pergunta e me mandou cavar.
Eu encontrei um excelente ensaio sobre o filme escrito por William Beard, e aqui está um trecho que pertence à sua pergunta:
Se isso era demais para ser recebido, em poucas palavras, Munny havia sido perdoado por sua esposa, sociedade e, mais importante, por si mesmo - mas a atração de seus velhos hábitos é muito tentadora e ele regride no mundo violento que ele tinha deixado para trás, acabando por destruir os últimos vestígios do seu "eu bom" através da morte de Ned. Em última análise, ele percebe o que ele é, o que ele sempre foi; imperdoável.That he should get away with killing while Ned dies horribly for not killing creates the moral abyss into which Munny plunges in forsaking his “good” self and embracing again his “bad” one. Here lies a way to an interpretation of the film’s cryptic title. Munny’s wife Claudia, in attempting his regeneration, in pulling him out of the maelstrom of nihilistic compulsive violence and drunken self-obliteration into a world of principle and language and family and human self-recognition, forgives him. The act of forgiveness produces the (feminine) redemptive result of self-forgiveness. In addressing at last the buried consciousness of horror and guilt, the fiery cycle of repression and violence whose first victim is the perpetrator is broken, and the functional person William Munny (the “good”) is dredged up into view.
Once established in the social world of human relationships, gainful occupation, the code of civility and “decency,” Munny is happier than before. Even after the death of his wife, and despite the rather naive and rudimentary nature of the precepts upon which he leans, he continues forthrightly in the same path. The process which pushes him back off that path begins with a condition of economic hardship and the unfulfilling nature of his labour. Pig-farming is dirty, frustrating, humiliating, and profitless. The temptation to move into another form of paid work—killing for hire—is very strong, when that work suffers none of the drawbacks just enumerated. In drawing Ned Logan into the business, Munny wishes not only to provide himself with a dependable co-worker, but to give himself a degree of orientation in this strange endeavour. Ned, like Munny (and like the Eastwood persona too), is a former hellraiser, now a respectable freeholding family man.
As the film proceeds Ned develops into Munny’s anchor to the world, his reassurance that he has forsaken the old ways (which Ned also witnessed), and his guarantee that his actions have some foothold in a worthwhile life-pattern, in decency and fellow-feeling. But Munny makes the mistake first of returning to killing (however different his motives this time) and second of pulling Ned with him. When this happens the results are different from what was anticipated (this too is morally instructive). It is Ned who is punished for the transgression, a transgression he did not truly commit; Munny does everything and goes free, and gets paid to boot. It is not just that any notion of a higher system of justice and moral equilibrium is derisorily contradicted by this development. The death of Ned is also Munny’s personal loss of his “good” self, his loss of Claudia’s forgiveness and his own self-forgiveness. When he walks into Greely’s to kill Skinny and Little Bill he is a creature who has lost salvation, a damned soul, “unforgiven.”