TIFU by trying to humanely kill an animal

This happened like a half hour ago and I need to talk about it because I'm not ok. I live in a pretty rural area. I had just gotten home from the vet with my cat and noticed the neighbor's dogs were in my yard for some reason. They were both just laying in the snow gathered around a dark spot, couldn't quite tell what it was from that distance. I called them over and I was able to grab the friendlier dog's collar. My neighbor came out, apologizing, and the two of us wrangled them back into her fenced yard. Curiosity got the better of me and I went to see what the dogs were so interested in. I found a bedraggled, wet looking 'possum. There were some signs of a struggle, little spots of shit and piss in the snow. Then I noticed the shallow, labored breathing of the poor creature. Oh fuck it's still alive. I ask my neighbor if she has an axe or something, the dogs got a 'possum and I want to put it out of its misery. She comes back out with a shovel and asks if that will do. So. We are starting to enter the fuck up territory. I tried to cut its head off with one really hard jab, but no dice, and now its convulsing in a disturbing way. So I sort of set the shovel on its neck and I jump on it like you would dig tough ground, and still, my 220ish pounds were not enough to seperate its head from its body. Its rear half lifted off the ground and i saw all of its insides move all the way up and then all the way down. I felt like i was going to vomit and I'm waiting for intestines to erupt out of its butthole or something equally horrifying. Then somehow a simple phrase passed through my mind: "playing 'possum." Oh shit. What have I done? On the verge of tears and finally realizing that I hadn't actually seen any dirsct evidence that the animal was injured before I got there, I bring the flat of the shovel down on its tiny skull. It thankfully and finally stopped moving and I saw blood coming from its ear so I knew it was over. I gave my neighbor back her shovel and she gave me a garbage bag. Now I'm just hugging my pets and children and feeling awful. Sorry about formatting, on mobile, might fix it later.

TLDR; TIFU by trying to have compassion, maybe did a murder instead.