Yes , renal failure from a urinary infection can be fatal if not treated by a veterinarian .
Yes, they can. Tigers and lions do so. However, it is less likely for house cats to do that.
Cats probably CAN kill you but nobody knows. If they scratch your eye out, that can kill you (or you end up getting a glass eye.)
If it is deemed necessary by a vet, that vet will do it for you.Anything else is bound to be a cruel and inhumane way of doing it, thus illegal.
First off why do you want to know that?