First, consider Max’s motivation in snagging Deborah. Throughout the film, Max is driven to sexually possess the women Noodles has already had. During their initiation into the world of sex, Max insists that Noodles take Peggy first. Years later, Max takes the woman Noodles raped, Carol, as his lover. He even asks the prostitute in the hearse who gives Noodles “a roll” if she wants “a little pick me up.” Whether this indicates Max’s latent homosexuality (quite possible considering he constantly refers to sodomy in his language, including during an early exchange with Noodles: “Drop your pants and I’ll stick it to you again.”) or merely stems from macho competition is open to interpretation, but one thing is clear: Max desires Noodles’ women. And possessing Deborah, Noodles’ life-long romantic ideal, would be Max’s crowning achievement. He must have her and will go to any length necessary to make it happen.
Deborah might have been receptive to Max’s initial advances partly out of revenge against Noodles for raping her. But more importantly, Max, unlike Noodles, is the kind of man Deborah would respect. He may have started out as a poor immigrant, but, unlike Noodles, he is a man of ambition with big plans for himself. Max reinvents himself, literally assuming a new identity, and becomes just the kind of successful, upwardly mobile type that would attract Deborah. Noodles himself points out the similarity of the pair when he complains to Deborah, “You’s twos are just alike – that’s why you hate each other so much.” Deborah and Max, for all their apparent antipathy, share the same values for wealth and fame – in Deborah’s words “making it to the top.” By contrast, Noodles is a self-described loser whose lack of ambition is held in contempt by both Deborah (“you’ll always be a two-bit punk”) and Max (“You’ll carry the stink of the streets for the rest of your life”).
Just as Deborah is willing to give up the one person she’s ever loved to pursue her dreams of stardom, so Max is willing to sacrifice his boyhood friends to facilitate his own ascendancy. Both Max and Deborah, who is portrayed as narcissistic prima Donna, are willing to do whatever it takes to “get to the top.”
Plus, the fairy-tale world Leone creates is so self-contained and hermetically sealed that the central characters barely interact with anyone other than each other and those immediately associated with them. In this context, who else was Deborah going to end up with?