Something about "birds of a feather flock together..."
Ofc, it was the "yandere" ending. And you know what? It's a "good ending", so to speak.
I kinda feel for Rio...? But it's more in the fact that she could have corrected her whole personality from the very beginning, beyond the main story and this spin off... But, oh, well. At least they are happy together...? (she didn't deserve Mahoro and friends' friendship. At all. At least, all this cheating situation with Rio made Mahoro realize that Rei was a scumbag).
(I still wish we could have gotten much more psychologic introspection about Rio? Although it was very obvious that all she wanted was "validation" and feeling superior to her "friends". Sure, that still wouldn't change anything... but... I dunno, that would have been more interesting than "yup, those together are still together. Don't need to feel sorry about them... they are beyond therapy help. Be glad they are not in Mahoro's life anymore").