Fwiw, it was quite a bit more than "a little sad" (nobody gives a better haunted look than Murphy) Regardless, even outside off Japan, I think it's clear that the movie doesn't do nearly enough to hold Oppenheimer to count for his own actions, spending the bulk of the runtime more concerned about his PR than his creation. Which I get is an interesting "angle" on the whole saga, but is acutely tone deaf to said creation's real world impact.
While well made and well acted, the story of the movie (and I assume the book as well, though I haven't read it) seems more inclined to forgive Oppenheimer for the horrors he unleashed, which I'm not sure even he would give himself if the movie portrayed him even somewhat accurately. Suffice to say, I'm not surprised to hear Japan finds the whole thing to be just a bit questionable.
There is a reason for why Nolan focussed on Oppenheimer's psyche rather than the weapon. Because the Nazis already split the atom.
It's only a matter of time before someone build the bomb. Which if not for Oppenheimer and Einstein would most probably be Nazis.
Einstein the most pacifist person of the time went to the president to request him to start the Manhattan project.
You can see this was evident from the fact that so many countries independently developed their own bombs in the decades that followed and USA itself building hydrogen bombs without Oppenheimer even though he came up with the idea.
Oppenheimer didn't play any special role in building the bomb. He just sped up the inevitable.
When Manhattan project was started it was an attempt to beat Nazis and build the bomb before they could. But when it was finished Nazis were already defeated.
The only thing wrong was to drop it on Japan and it's not Oppenheimer's fault.
If you look at it he did managed to end the war once and for all. The reason ther is no third world war is because everyone is scared of the hydrogen bomb.