The villains had the goal of driving back the fungal forest, which was advancing and destroying civilization. They didn’t know that it was filtering pollutants out of water, and they were using militaristic techniques that have the designated “evil” tag but that IMO should be considered in a more nuanced way.

Basically, Nausicaa thought she was in the post-apocalypse and wanted to “live in harmony” with the state of the world. The Valley of the Wind had unique advantages allowing them to do that. Kushana thought the apocalypse was still ongoing, and it was - we saw that other settlements that didn’t have the special meteorological layout of the Valley of the Wind were being overrun by advancing fungal forests.

From a very slightly different perspective, Kushana was the hero here. She was unifying a disorganized and doomed rabble of survivors to work against their doom, and recovered an ancient superweapon to turn it into a protective force that would save humanity instead of attacking it. She turned out to be wrong about a few things but being wrong doesn’t make you evil. I’d argue that she wasn’t really wrong about some of the important stuff and the movie ends on a mixed note.

It’s been a little while since I’ve seen the movie, mind you, but it stuck with me pretty strongly.

Unless I’m mistaken, they’re talking about Nausicaä of the Valley of the Wind (1984)