It’s been a while since I saw this so I’m due for a rewatch, but knowing Hitchcock’s output in the late 30s-40s I can guess there’s a deliberate anti-Nazi theme here. The murder gives Rupert a brutal awakening at how the Übermensch theory becomes perverted, just like the Holocaust gave America & Europe a brutal awakening.

Before that, in society as well as Rupert’s mind, the Übermensch theory was gaining traction. It’s one of those things that sounds impressive on paper but doesn’t work in practice, as Tolstoy’s Crime & Punishment warned 100 years earlier.

To drive the point home, Hitchcock, like Tolstoy, would want to show an actual awakening in the protagonist where he realizes his error and changes his mind. Otherwise the message wouldn’t be loud & clear. Anti-Naziism is one of the points where Hitchcock threw subtlety to the wind and gave us a morality tale ending. So with that in mind, I think Rupert’s reversal was meant to be taken as absolute. He really did change his mind, just like most of society did (or should have) after learning about the Holocaust.

I do remember some great nonverbal acting by Jimmy Stewart when that moment clicks. You can see the sudden humility in his face, which wouldn’t be the case if he were still clinging to his original stance.