In Season 1, Kes was reliably lovable and caring. Then, in Season 2, the writers started exploring the darker side of her personality in "Cold Fire," an episode that foreshadows "Fury" in many ways. Season 3 gave us further steps in Kes' unpleasant transformation: "Warlord" and "Darkling."
Nevertheless, I concur with you that Tieran had full control of Kes when she treated Neelix so shabbily, because even by Season 3, Kes had not become that bad on her own. But if it was Tieran who did the deed, then why didn't Kes undo the damage and resume the relationship with Neelix? The character is irreparably damaged, no matter which theory you support of who was in control.
I see many flaws in your statement.
I see nothing in "Cold Fire" which in any way foreshadows the s**t episode. On the contrary, in "Cold Fire" Kes is tempted by evil forces. But due to her strength and willpower, she overcomes that temptation and saves the ship at the same time.
In "warlord", her mind was taken over by an evil entity. But she fought that entity all the way and with a little help from her friends, she gets rid of that entity.
And what on Earth did she do wrong in "Darkling"? Dis she control The Doctor or what?
As for the break-up with Neelix, it's all down to bad writing. Braga and his clown gang were never good in writing about relationships, not in TNG and not in Voyager. Everything they wrote became stale and lifeless. In this case they didn't even manage to come up with a decent break-up scenario. Even I could have done that much better. They were the ones with darker sides which they clearly showed when they deliberately gave the finger to the Kes fans with
that episode in season 6 and when they insulted the J/C-ers in "Endshame".
As for Neelix himself, he was a nice fellow and I really like him!
But he wasn't always as nice and pleasant. He, if anyone had darker sides, most likely the result of his tragic past. When it came to Kes, he was jealous, over-protecting and overbearing in the worst possible ways. Any normal woman would have got tired of those antics sooner or later.