Why didn't they kill off Kes?

It stems from the natural aging process. In Before and After we see that as part of the aging process they loose their memories. This is what was happening to Kes. Her memories were starting to break down which caused her anger and confusion

 

I forgot about that. Although, it's funny that her memory remained intact enough for her to seek out and find Voyager. Convenient huh? I guess that needed to happen in order to have this episode happen at all

 

It was probably just starting to break down so she had some memories but they were distorted

 

People comment about the Borg being overused. If Kes had been killed off, Voyager might not have been thrown beyond Borg space. Then they'd have a legitimate reason to use the Borg as much as they wanted, for however many seasons it took for Voyager to travel through Borg space.

 

I respectfully disagree. In Before and After we see that the dementia did not start until she was well into her 9th year. The birthday party was for her 9th birthday. At this point she's looking older than Tom but still has her mind about her. But since Andrew is not full grown, I assume he's less than a year and she looked young when he was a baby.

Kes may have been 8 at the most when she returned in Fury unless the ascension/descension accelerated her aging process when she returned to a physical form. Also, why would an ascended energy being (even if they could retake their physical form) go senile anyway? There's no physical brain cells to break down to cause the dementia until she did re-take physical form. Since she could appear both young and old. Why would she take on a form that had dementia?

Besides didn't she say more or less that she wasn't ready for the things that happened to her when she ascended?

 

I don't know. Admittedly, I forgot about the memory loss thing but was it established that 100% of all Ocampans lose their memory as they age? (I don't know. I'm asking) Besides, even if she did suffer from memory loss, the outcome would not necessarily be resentment and violence toward her former friends. I still think the lack of explanation for Kes's actions in Fury is a pretty big plot hole.

 

I just figure that her "gift" took some energy from her so it accelerated her aging and memory loss

 

I think it can be fairly easily explained with just a little thought. I find it easy enough to see how it follows before and after and cold fire. So for me it's enough to just enjoy the episode