Heart murmur? Is that a diagnose from the vet? What do they say? If as you say he is not suffering why would you put him to sleep? Better try to find out more about this mulfunction and consult a homeopath maybe. Let him sleep beside you instead, in the meantime try to find a cure. It must be the way out thinking that terminal cancers are curable. And pay attention on food you are giving him to eat. No processed food as it is death food. Only organic.