WHAT KILLED MY FISH???

Two days ago, I discovered that my oldest fish was dead in the back of the tank. I would have put it down to old age if it wasn´t for it being CHOPPED IN HALF!! Straight away I though... Crayfish! But it was really wierd, the Crayfish hadn´t ever eaten a fish before. It gets angry at the big ones sometimes and one cut one of the Gouramis tenicles but nothing resulting in death! About two months ago another one of my fish died of old age, I saw it dead but decided to get it out later because I was in a rush, but when I came back it had been eaten, most probably by the Crayfish. Do you think the Crayfish killed the latest fish or just ate it once it was dead? If its killing fish it will have to be put in a seperate tank but this would cost money. What should I do?