Necrozma was pretty much an evil legendary. He literally just stole light for no other reason then he wanted it, and almost wiped out an entire planet because he was angry with them. Technically they started it, and it's pretty much canon that stealing the "light" from a region only takes away literal light, and not the warmth of the sun, but still, that's not cool.
Another thing, Gigantamaxing sounds like the equivalent to Megas or even Ultra Burst. A special form of a Pokemon that makes them bigger and more powerful? That sounds like Mega to a T.
A lot of this sounds really interesting, but an evil team that's well-meaning? We better have some other organization like Aether pulling the strings again. If I had to bet, I'd say Leon is behind it all, like Lusamine was, and his ultimate goal is to capture the Legendary so that all of Galar will love him as their Champion. My guess is that as he gets older, everyone likes him less and less, and he craves the spotlight of the Championship again, so he's trying to use the Legendary (whose bio says that they "captivate even their opponents with their battles" so Galar will hail him again.