If I'm honest, I don't agree with stunning as such either and I'll say why.
Thinking about my life, or any life, when it comes to a point of inevitably how would you ideally want to go? Since this is a subject close to my heart and witness it all too often, I feel more qualified than most on this subject. (Not arrogantly I hope).
The 'subject' shouldn't linger.
The subject shouldn't be aware what is to come.
The subject should not suffer.
As I said, animals know what is to come when they hear the cries of others. So I would prefer a situation where an animal is going about its business that it's known all its life and then - END!
No stun which they may, for all we know, not be completely 'out of it' or botched! No communication of impending doom. And absolutely no suffering, mentally or physically!
It's no more or less than I would want and hope for to 'exit' from my own life. Why should any animal deserve less? It should not be beyond the wit of 21st century mankind to do this, instead of barbaric and/or religious/sadistic practices!