I have the vague feeling like the last 40 years of movies must have contained at least one scene where a villainous figure makes conductor-motions as things explode to music, but I can't recall anything specific.

There was the V For Vendetta movie where landmarks exploded to the 1812 Overture, but no gesturing was involved.

It's not just V For Vendetta, 1812 Overture is supposed to include explosions (artillery/cannon fire) in its composition.

P.S.: I was wrong, the ending scene didn't have that because [spoiler omitted], but there's a much earlier scene [0] that fits the bill.

[0] https://www.youtube.com/watch?v=VCaT6tU7V8Q

The forgettable “Virtuosity” had Russell Crowe doing something like that, but it was awful.