I have 3 cats, the youngest we got about 6 months ago. Our oldest cat was not happy but tolerated her up to now. All of a sudden The eldest is trying to kill her, to the point where the youngest is urinating and defecating all over the place. I have a feeling that the youngest is unstable, we found her on the side of the road so I assume she comes from a line of feral cats. Being a former animal control officer, this sounds to me like a mental disorder. The youngest has a completely different disposition, and almost seems to not know where she is anymore ( she still responds to her name though ). She has to be separated from the oldest because she(oldest) is hellbent on getting at her, she seems to act as if the youngest is a new cat just brought home. Any suggetions would be great, but we are probably going to get rid of the youngest. We no longer want to trust her( it's the way she looks and acts, she almost seems as if something snapped and the oldest knows senses something that we can't )
By the way in case I wasn't clear, the youngest has been fine and friendly up until now, but she always picked on the oldest. Do cats hold grudges? because it seems as if the youngest finally ticked off the oldest for the last time and will not leave her alone until the youngest is dead
Ryan

Someone else on this site may have a better answer than I do. But, I will make an attempt because all of my life, I have had cats for pets.

If a new cat is brought into the home where there are already other cats living, then there is going to be a new play between the cats for who is the dominating one and for territory. The situation may settle down after those issues are resolved between the cats.

Another possibility might be that the older cat does perceive the new cat as being defective and wants to get rid of it. Not sure about that, though.

I have been witness to two cats that had been living together for many years in a friend's home. For some unknown reason, one of the cats just started to attack the other repeatedly. It just continued to go on until the aggressive cat just had to be removed to another home.

Cat snapped!

I would say that your cats need to be separated and you need to give each the same amount of attention. If the little one was bothering the oldest - there may have a power struggle and your oldest has had enough - and is now really angry. If the little one is urinating and pooping - she is really getting beat up and is so scared. I would not bring them back together all at once - I would do it gradually. I have a family of 6 cats - and believe me I have had to deal with some issues. The biggest one, Mouse, is 18 pounds and a real jokester. His sister Isadora had enough of his tackling her one too many times and one day she went crazy! She was screaming at the top of her lungs like someone was going to kill her. I was outside, 2 houses away one time when I heard her screaming inside the house! I kept them apart and Isadora eventually calmed down but it took about one month.

If you suspect the little one is ill, you might want to find a vet that can check her out to make sure she is all right.