can anyone tell me why a bitch eats her puppies.
i got home last night to find one of the pups rear paws half chewed off.
is this normal or do i just have an idiot of a dog?

What a shock for you :(  This is one reason not to leave a bitch unattended with ehr pups until good maternal behaviour has established.

This is not normal, but not unheard of.  Some bitches get carried away with the ckleaning and that is why they do it not differentiating between umbilical cords and membranes and limbs or ears.

Some bitches are also convinced they have done something dirty adn want to hide the evidence. 

This can also be a sign of Eclampsia where the bitch becomes very distressed, staring and carryng her pups around or kiling them.  this is a serious metabolic disorder where the Calcium supplies drop to dangerous levels.  Please take her to the Vet to rule this out and of course to decide what to do whith the pup or pups.