They should use the exact same method they use to euthanize strays at the shelter.
If it’d good enough for innocent animals, it’s good enough for killers.
Bring back below-ground nuclear testing...?
Instantaneous: one would be ionized before the pain signals have a chance to propagate from the extremities to the brain...
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.