There isn't much of one, really. Rather, his response to the figure that represents the planet's most notorious instance of institutionalized slaughter is pure fascination. That old black magic of genocide that they weave so well ensnares the boy, too. Mesmerized, he essentially blackmails Dussander into an education in the theory and practice of Hell. And, as played by the great British actor McKellen, Dussander is quite an apparition.
Singer is onto something here.
The theatricality of Nazism was its one triumph, and in its artful manipulation of icon and symbol, it retains the power to beguile to this day, particularly with the lonely or the isolated, who are vulnerable to its catechisms of natural superiority and the freedom of living beyond restraint. This is something Singer can't bring to life. Young Todd is simply drawn to the black creed of Nazism because . . . It's not satisfying. The old man gains the upper hand and blackmails Todd into becoming his lackey.