The ex-Tory MP’s documentary How to Kill a Human Being convinced Oklahoma – and then Alabama took up the baton

Shortly after Alabama last week carried out its second-ever execution using nitrogen gas, state officials took credit for pioneering what they see as a breakthrough approach to the death penalty – even though it has sparked outrage and revulsion among critics.

But the idea to execute inmates in the US with nitrogen was actually set in motion by a rebellious and ultra-conservative lawmaker in Oklahoma, and surprisingly involves a former British Conservative MP turned television personality, Michael Portillo.