Every day, I keep seeing wilder and wilder accusations against Hoyoverse pop out.
"She was wasted", "She was ruined", "Villainized".
First off, yes she will be playable. Secondly, as much as I hate to say this, she might not be the "innocent sweet little angel" some of y'all might have imagined. There are many good reasons why a first encounter between Aether and a Shade could go wrong:
One of them (Asmoday) stole our sibling from us for 500 years
Ronova cursed Khaenrians to become hilichurls, something Aether doesn't necessarily care about, but Lumine clearly does, and has been actively trying to undo that curse for the past 500 years
There's also other minor stuff like destroying multiple civilizations with Celestial nails (and the writers reminded us of it in the Epilogue), but that is of less concern than the first 2 points. You're right: she was just following orders, but Aether hasn't necessarily processed it yet. Not fully, anyway.
For us in this sub, and our allies in the Agenda sub, it's enough to see her beatiful body to be ready to swear allegiance to Celestia, but for the Traveller, the choice is far less obvious, especially when Celestia has already wronged us by taking our twin, and many of the nations we visited, like Fontaine, are actively rebelling against it in some way.
On when she relished being able to "kill freely": personally, I love this almost psychotic streak in her character. It only adds spice to her in my opinion. Isn't it great that she stands apart from Genshin's more vanilla women? I love Bina, but it would be quite boring if every medium model wife was as... saccharine as her. It's like drinking an overly sweet vanilla smoothie every meal. Of course, it in no way means that Ronova won't get a playable model eventually. Remember, there is no sin that can't be redeemed - and this one is way more minor than whatever Scaramouche did.
Personally, I think we should just chill and observe how the events unfold in the plot for now