yes, i 100% understand. Stunning doesn't always work. You can see it in human executions: the substance that is supposed to make person fall asleep doesn't always work, amount si too small or one giving the substance doesn't know the correct methods, which leads to quite horrifying results when other stuff is given
But that is exactly the problem: we still kill other humans regularly, the cruelty like that is completely fine to us, especially in countries where death penalty exists(just look at what US has done recently and how they seem to think about stuff) and not only that, many countries who have had it removed have arranged public votes on it, and in many of them people want it back. It's is a state of mind this world has right now, that killing, as long as there is at least some reason, no matter how stupid it is, is justified. Was it humans, animals or plants, we don't think about it as long as there is a good reason that applies to us personally. Pigs are killed for food, criminals are killed because they are dangerous, trees are chopped down because we need space for crops, etc. No, those pigs do not need to be killed, nor do criminals or trees, but do we care? of course not. We are cold-hearted by nature, and those who aren't, well, i'm sure you know what society thinks of them(i don't mean this as an insult, sorry if it comes off that way)
And talking about companion animals, i have seen animals killed for varying reasons. My grandpa got his dog and cat taken down for simple reason that dog was getting old(she had no health issues yet however) and cat couldn't do his's job, hunting pests, anymore. My dad's rottie-dachshund-mix was killed because she had bad tumor in her belly that had spread and caused her horrible pain. and yes, they didn't need to be killed so soon, but the way it was done was painless, which makes it at least little better, and in the case of a mix, she was saved from going through unnecessary suffering. Yes, i miss them, more than any human relatives i have who have died in my small lifetime, but i'm fine with it. at the very least, they don't need to see the horror that this world is turning into.