I'm very sorry. That is really a tragedy; not only because of your pet's untimely death but also because it may make more people fearful of neutering their pets.
Obviously there was a problem with him prior to the surgery. I can't say exactly what (there are a couple of diseases which could do this) but it caused him to have an inability to coagulate his blood properly.
Please do not feel guilty about what happened. You did nothing wrong. If the vet wasn't able to detect a problem prior to (and during) the surgery, then YOU definitely could not have detected it.
I'm sure your pet had a great life with you, even though it was such a short one. You did the right thing about getting him neutered. I hope all his memories stay close.
