20 plus years ago at the Garland, Tx animal control they used a vacuum chamber, and I suppose they still do.
It took only 3-4 seconds. But the bodies were severely distorted/collapsed/shrunken in the process, so that may not be acceptable for a human.
It’s either humane or it’s not.
If it’s humane to use that method on innocent animals, then it should be humane enough to use to murderous scum humans.