Why would one of my cats KILL another one of my cats?

Question
I started out with 2 cats that were homeless kittens that were approx 3 mos old and after having them for several years I took in a female feral cat that I eventually tamed over time and had her fixed and she began living in the house with the other two cats.  My female cat didn't like this other cat at all.. the male cat liked her and they got along very well..but my female cat would growl and chase the new cat away all of the time and to this day.. she still does not like this 3rd cat. and Now.. it's approx 2 years later and it's winter time... a cat showed up at my door along with some other feral cats that I feed outdoors and she kept coming in as soon as I opened the door.. I could tell that she had been someones cat.. she was tame and sweet but very thin and starving and the only thing that I could tell.. was that someone must have dumped her... I live in an area where the housing people were not allowed to have animals and she was always out there freezing in the cold but not hanging with the feral cats that were always hanging around each other.. so, I finally just let her stay in and join the other 3 that were my house cats.. rather than let her freeze to death outside. I started out by keeping her in a cage so that the other cats could see her..smell her and they could all slowly get to know each other... and after a couple of weeks I started letting her out to eat and showed her where the litter pans were.. and after a hour I would put her back in her cage and I would gradually let her stay out for longer periods of time until I let her stay out in the house like my other cats. I would say that everything was going well for about 6 weeks but I noticed that she didn't interact with any of the other cats.. she would avoid them and stay in areas where they were not... and last evening around 5pm I heard a HORRIBLE scream from a cat and I looked and my female had attacked her and had really hurt her... it was a SCREAM that I have never heard before and I saw my female hovering over her and the new girl was shaking and was trying to walk but her back legs were not working so I tried to help her and she was having a terrible time.. I put her on the couch and she fell off while trying to get her back legs to work.. I didn't see any blood but she has very long hair that is very thick... then all she would do is shake and pull herself around with her front legs.. I put her in the cage that was hers before and hoped that she would get better..  I didn't know what to do.. all of the vets were closed and I had just hoped and watched over her but she started having trouble breathing and finally she had some seizures and screamed out as if she was in TERRIBLE pain and she was DEAD!!  I have NEVER heard of anything like this EVER happening.. I have seen feral male cats fight over females and they sounded like they were evil demons and like they could kill each other.. but, I've never seen that ever happen and I've never heard of such a thing from anyone in all my life.. I even tried to research online to try and find an answer and I just can't..  Why would my female attack and KILL this cat like she did.. and will she do it again.. now that she has done this?  I am just beside myself.. it's like a horrible dream and I just can not comprehend this at all. I took this cat in because it really needed to be rescued... or it would have died... only to have it be killed by one of my very own cats.  I know many rescue people that take on another cat or two a year and they have never had this happen to them.
What could have possibly happened that made my female cat kill this poor helpless sweet cat??

Answer
Vicki,

Although the untoward event is not common, it does happen.  When a cat chases another cat and does catch up, it would tend to pounce and bite on the back, usually towards the tail.  I am not a vet, but from what you describe, it sounds as if the bite severed the spinal cord.  While this was rather a nasty bite, the intention of the biter was dominance in her territory, not to kill the new cat. It just was chance that the bite had the effect it had!!!!!!

I am so sorry that this happened, but my guess is that your female is telling you she would rather not have another cat in her territory.

Sad regards... Norm.