I agree with hissy that the cat should be checked for worms. If the worms are 'showing' the infestation is pretty far advanced. Your cat can pick up a lot of parasites from the ingested prey.
The soft paws, at least, appear to be working for you if not for her.
Unfortunately, if you leave cats outside permanently this is what happens. By their very nature, cats are killers. If you dont like it, then take the cat indoors and only allow him limited access to the outside - ie when you're at home. Our cats only go outside into the garden when we're home. What's the point of having a cat as a pet if you're going to have it living outside?
Well, it appears I have set of a fire bomb with my serial kitty killing debockle. Those of you worried about worms and infections and parasites, please worry no more, the cat has been seen by a vet and given the All clear. The roomate who is the actual owner of the cat, has informed my mother that she will be moving out after the holiday weekend into an apartment. Apparently it is an upper floor apartment, so...the kitty will no longer have access to the outside world. Soft paws website was sent to my mother per request and she has read it and is doing her best to keep the kitty indoors until the move.
So for now, the Serial Killing is over. Thank goodness.
I once again thank everyone for their opinions and thoughts. Especially the suggestion about the electric doggie door. That is something my mom is seriously looking into now.