Lynx I do understand what you're upset about. I was rather incensed at the treatment of a VOY hero of mine in a certain book:
Your point about a long time character versus the dumb/evil get-rid-of demise of a short term character is good too.
As to what was going on in Fury.. I had some ideas last time I watched that ep (which was fairly recently). Firstly it highlights how we know NOTHING about Ocampans and how Kes herself knows very little about them. They are a stunted people as they exist on their own planet. They have enormous potential but when Kes begins to manifest that potential she has no role models, no idea what will come next. No training, no guidance, no clue.. imagine a Vulcan going through Pon Farr in a cave by himself without ever having heard about what it was or seen it. He would go insane! And might well try and kill the first people who approached him because he had no context for what he was experiencing so was highly threatened.
So Kes experiences changes for which she has no preparation and which terrify her--her lashing out and blaming it all on her family is typical of what adolescents go through--X 1000. For all we know it WAS a kind of adolescence, a maturing from youth into something greater. Obviously there is more to the Ocampans than the nine simple years Kes originally thought her lifespan would cover.
So Kes's response to her changes is much worse because she has no cultural context for it and it is terrifying. Her lashing out would not be as much of a problem if she was lashing out at other beings such as herself.. but she has no peers and the only ones she can blame are the only ones she has been in any kind of relationship with. Unfortunately they are in no position to rein in her anger because they are so weak.
I think you explaination makes perfect sense in that fact that it's also real world believeable. We also have to take into consideration what was stated in "Before & After" and how we Ocamians get older the "morelogium" sets in, a period when their memory starts to break down.
Plus I don't in anyway believe TPTB ruined Kes and certainly not on purpose. Voyager was the flagship show of UPN, a brand new network. The show was already on a decline in viewership. Why would they ever wish to loose more by purposly alienating more fans by intenionally ruining a character? That doesn't make any sense logically or business wise. Loosing more viewers also hurts the show and Paramounts wallet.