I'm very sorry to hear your sad story. I suppose drugs, whether used for humans or for animals, can cause harm as well as good. Vets have to weigh up the pros and cons, and sometimes they make misjudgements or are simply careless, so as other people on this forum have suggested, maybe you could put in a complaint to their professional body. I hope you will find comfort in the fact that you loved and cared for your dog for a very long time and you did your best for him.