Oh nooooo! The male love interest is dead and the heroine is doomed!
The author must really think we're stupid at this point to even waste our time with this nonsense.
But seriously if Isekai girl being a traitor is real and not some kind of fake out I am 100% done with this series.
Not because I was a big fan of her. But because the author just straight up lied to us about her own internal monologues so we wouldn't see this twist coming. It's a clear sign a writer is bad when they lying to readers makes for a valid plot twist. So if that's seriously what happened here then I'm just done because it will mean NOTHING in the story actually matters. The author can and will change the facts on a whim in the name of a twist so nothing established in the story has real meaning.
A good twist is one where the hints were there and could logically come to the result that is eventually revealed. But you still didn't see it coming because you interpreted the hints differently.
But there is still a chance. A chance this is just a REALLY REALLY dumb fake out and she'll turn out to still be on the heroine's side. But if not I am just dropping this. It was a good run but no more.