Not always instant.

Solution:

Affix 3 captive bolt pistols (cattle execution gun-sticks) to a "death helmet" with synchronous triggers. Even if one or two fail or are not quite ineffective, the odds are that it will get the job done.

Also, after, have the chair tip backwards to dump out the naked corpse into an industrial meat grinder over a fast flowing river for an eco-friendly solution that nourishes life in the river.