Hmm... Well if it HAD to go in the "evil force possesses her" route by the end, I'd rather the bad thing (I'm just going to call it NMM) was presented as more of a temptress than something that aggressively took control. Egging her on about how much ponies hated her and her night until she finally agrees to accept it of her own free will, even though she clearly knows better and recognizes that NMM doesn't mean well.
That way she's at least an active participant and thus morally culpable. Your way still casts her as a victim.
But I'd still rather she just snapped on her own.