>humane killing

Please explain how taking away another living, breathing creatures life can be humane.

Is picking a flower or harvesting wheat inhumane? If not, is there something special about the process of breathing that makes animals more worthy of protection than plants?

I’d argue it’s the presence of neurons, brains, nervous systems that make the difference.

Sure. We can estimate the level of pain caused, and minimize it. That's the definition of humane; it's totally arbitrary and it's up to us to define. Nobody said zero pain or suffering is on the table; it's not like it is an option in our own lives, either.