'The only thing necessary for the triumph of evil....
is for good men to do nothing".
That, dear friends, is the core message of this fine film.
Actually, it's not. The message is that almost anybody who considers himself or herself a good person can be led step by step down a long, shallow slope into a morass of evil. John Halder doesn't do "nothing." It's just that when he acts he makes the wrong choices, for whatever seem to him at the time like good reasons. His hallucinations about men playing or singing Mahler keep pointing that out, but he pretends not to understand. As he makes wrong choice after wrong choice he becomes stronger in his way of dealing with the world, but at the expense of virtue and humanity.
At the end when he sees the prison orchestra and realizes finally not only that the music is real but what it means, he weeps. Too late, of course.
The original play and the film are meant to remind us that we can all take the path of small steps to evil ends.
The filmmakers wanted especially to make this film during the last Bush administration, when so many unsavory things were being done by the U.S. government in the name of fighting terrorism.
I think you and the OP are saying the same thing essentially.
Edmund Burke's quote shouldn't be taken literally. 'Nothing' doesn't necessarily mean literally doing 'nothing', but can mean doing, as John Halder does, the wrong things and rationalising them rather than taking a proactive stand against an evil regime at the right time.
Halder knows deep down that the Nazi Party represent everything he is fundamentally against, but he complies with them, and tries to rationalise his decision to join the party to his friend Maurice, because he'd rather bury his head in the sand than face up to the natural consequences of their malign philosophy. That is in essence an example of 'a good man doing nothing'.