The thing here is that skin lines exist to present a thematic for a champion. There are champions we want to see IN THAT THEME. Having a "twist on a skinline" that you have expectations for doesn't do justice to the skin line. It only breeds disappointment. When you hear Xayah and Rakan are being added to Star Guardians, you fucking want Star Guardian Xayah and Rakan, not some edgelord purple "corrupted" version of Star Guardian Xayah and Rakan. That's largely the issue here. This is a popular skin line because of the specific thematic it brings. It's insulting and disappointing as a fan of the skin line to see them twist it this way, because it means that those champions have been denied a proper skin that fits into that skin line.
It's like going to a burger restaurant and asking for a medium-rare burger with cheese, only to have them tell you that they don't actually serve meat anymore, they only serve veggie patties with vegan cheese. It's not what you were wanting to get out of the experience, and it leaves you feeling like you've been cheated and swindled. This is the larger issue with these skins. Rather than make dedicated villains for this skin line that actually FEEL like villains in a magical girl anime, they instead decided to do what Riot always does - corrupt everything nice and good because they for some reason cannot tolerate having good guys be good. They're "villains," but they're called Star Guardians, so it's just a hollow façade of what players wanted from the Star Guardian experience.
Edit: Downvote me all you want, but there is a ring of truth in this. How would people feel if the reverse was true? How about the next PROJECT skins makes cutesy versions of the skins that all look like chibi characters that are pastels and bubblegum colors, and they talk in high pitched filters because Riot decided that "the Program infected the PROJECTs with emojis" or some shit like that. That's not what you want or expect when you hear PROJECT skin, so why is it any different to be upset when Star Guardians are going dark and edgy?