Really, that's how most of us want to deal with our predators. We're not out to torture or cause undue suffering to an animal. One quick shot and it's done. Most of the time, the animal never knew what hit it. I know, I know, there was another poster who did torture them by burying one alive and allowing another to starve to death. I truly believe, though, that most responsible animal owners would not do that. We have animals for a reason and that reason is not because we like to make them suffer.Not contradicting, but I find it odd that the possum would attack the dog for no reason. Possums usually only start biting when they're cornered or have babies to protect. Most of their hissing, open mouth gaping is a bluff, they want you to just leave them alone. They really (generally) just want to get away and go on with their business. Playing dead is an involuntary muscle response.
When I did wildlife rehab, my young possums would sometimes hiss and growl and open their mouths when I would reach into the cage. Once I touched them though? Snuggly buttheads who wanted their numnums.
Just for clarification (and I'm not judging either way this turns out) possums are very beneficial. They carry few diseases (although spell danger for horses) and eat a variety of pests, such as slugs, snails, mice, young rats, small snakes, etc. They're really fascinating animals, and they have their worth in nature. Their teeth are very sharp though. Generally (although obv not the case with OP) they are much shyer animals than raccoons, and aren't as liable to pick a fight.
I understand that you want to eliminate this possum, I do. I just ask that you please do it humanely and painlessly (as doing it any other way is cruel, and unnecessary, no matter how you spin it. No animal deserves to die buried alive for doing as it has for thousands of years.)
I really do understand protecting what's yours, and having responsibility for your animals. I get it, sometimes there is just no alternative to putting the animal down. But I'd like people to keep an open mind, and not completely villify the wildlife around them.