This YouTube video carries the title "Local Flavor TV: From Grass to Grill - Part 2 - Certified Humane Slaughter."
If you have the guts, put yourself into the cow's place and ask:
If like this steer you had not committed any crime, yet someone tricked and prodded you into walking into a device that closed down on your neck to restrain you would you call their trick humane?
If that someone then shot a bolt into your cranium while you struggled to free yourself (as did this steer), would you call him or his process humane?
If the butcher then hung you up by your legs and cut your throat while your heart continued to beat, would you call him or the process humane?
I don't think that humane slaughter can exist. I think the phrase provides a euphemism intended to put you back to sleep.