i'm sorry to hear that. If the throat was cut in a straight slit, than it was definitely a human. If there were two puncture wounds then it could possibly be a weasel or mink, they suck their prey's blood.
Where did you find the cat at? If it goes outdoors there are many dangers that can happen upon a cat that can slit its throat (falls from trees and hitting branches, fences, wires, etc). Sorry for your loss!
I'm so so sorry. If your cat found outside, then the cause could be tree, fences, and any other sharp things. But, sadly im guessing a human did this. A knife is the most likely, but the question is why? I'm so so sorry again, i hope this will help.
It would have been an animals claw, but it's more likely a humans knife, because animals claws would have more than just one slit/cratch. I'm sorry for you're loss, and I'd suggest maybe getting help to find out what or who did this to your cat. :(