You put a cat down rather than let her go naturally when he or she is in too much pain to even live relatively comfortably. You would also make sure with a vet that there is nothing else you can do for her.
You will need to put your cat down if it is suffering and in terrible pain. Some choose to let a cat die naturally rather than put them down. This is okay providing that it seems to be comfortable and out of pain. Unlike people animals cannot sustain a lot of pain killing drugs and it would be unkind to see them suffer terribly. You can be there when the time comes and comfort your cat while at the vet;s or home.