That is such a sad thing to have to come to terms with, but you should not feel in any way guilty about what happened. You were doing your best for your best friend - neutered dogs are much more contented with the constant urge to mate - you were doing right by him, and you had no way of knowing that your dog had a heart problem. I don't think the vet would lie to you on this. The same thing happened to one of my cats (although it was not a neutering operation, he was having a tooth removed). An unknown heart weakness caused his death, he was only eight years old. I am very sad for you to have lost your friend, especially when you came through such difficult times together. You will always remember him, and remember the love he gave you. Hugs.