Well, I didn't kill it.
I let the dog out again last night. She went nuts running along the fence. She kept standing on her back legs to see over. I went out with a flashlight to see what was going on. There was the opossum sitting in the woods 10' or so away from the fence. He just sat there. I had the chickens locked up and all of their food put away. Then I blocked off the gap under the gate with a 2x4. The dog and I went back out 3-4 more times. Eventually the opossum left. I heard it heading down into the woods and saw some brush moving. I guess it decided that invading my yard again was not a good idea. Hopefully that's the last I see of it.