How can I disable the video that plays behind the main menu? It's annoying.

Removing the Rise of the Triad/ROTTGame/Movies folder doesn't work (but it does disable the intro videos)

I think it's rendered in game.

How do I turn it off?

You are right that it's rendered in real-time so I'm pretty positive that there is no simple way to get rid of it. Your best bet would be asking at a place where the game's developers occasionally check.

Although I'm wondering why you're annoyed so much by the background animation.

I've already posted the question on the developer's forum. They haven't replied yet. The part I don't like about the background animation is the lightning. It makes it hard to see the menu and gives me a headache.

Lol, thought so. That's how I felt about the lightning during the first level.