Seriously, I really liked her as a character, cant tell you how many times I watched her battle for mewni scenes. Young Moon was such a bad ass. And then they do this shit to her. I mad.
My feelings when watching were like, they put her on Mina's side just to negate that feeling 2 minutes later, like they just pointlessly dragged her character into the mud. They made me hate her for no reason, so freakin pointless...
Now, with time to digest it, yeah you can somewhat understand her motivation. Mina came very close to yadda yadda berrying(kid friendly assassinating) Eclipsa. Moon found out, and came up with a plan to save Eclipsa life while also taking back the thrown for herself. Put Eclipsa in an unwinnable situation to force her surrender. We all saw how that went tho. But I think the problem was they tried to make it a big twist rather than dropping noticeable clues like a smile at the end of an episode indicating that Moon actually does miss leading people and having the yurt village following her orders has made her miss ruling the kingdom. Then while we would suspect it was her that made the solarian soldiers, the twist/surprise would be that she cant deactivate them and things would go pretty much the same way from there. The difference would be that we would have seen her intentions, while selfish, were at least based in trying to transition power with as little bloodshed as possible.
I dunno, I'm no witer, but they could have done a lot better.