My sympathy on your loss. Sometimes pets do kill each other. I have had this happen but cat to cat but dog to cat. I have to admit I have never heard of cat caticide. It is possible that some other animal, wild or domestic, may have done this. Now I would like to add one thought.
I worked with a local veterinarian who was into statistics. He did a calculation showing how much it costs to keep a cat outdoors. The amount was over 10 times the amount spent on vet bills by indoor cat owners. The numbers were absurd wheh he looked into extra costs for an outdoor unaltered males. This is just the financial end. The loss of life to preditors, children, and vehicles was heart-breaking and avoidable. Most of friends and members of our local welfare group no longer support outdoor cat keeping. I live in CT. and we now contend with feral dogs, coyotes, and an occasional bear. It is just not worth the risk. My wife and I lost a cat to a hit and run 25 years ago and we stopped letting any of our cats out off leash. We have since owned a great many cats who have lived more than 16 years, afew to over 20. It does not seem to bother them and they are truly safe from what can happen outside.
Once again, I am sorry for your loss it is never easy to lose a pet under any circumstances. Think about keeping them safe indoors. Peace and Love, Pop