When you feel up to it (or do it in writing- it's easier that way when the pain is still fresh) I think you should talk to your vet about what happened- & see if he has any ideas. I doubt you want to spend the money to do a necropsy, & it might not give you answers anyway- though since you have a number of other snakes, it might help (if possible) to be sure it was nothing contagious. But just see what he thinks might have happened? It also helps our vets to provide the best care when they get feedback & know the outcomes. So very sorry this happened. It's hardest when it's just so unexpected.