Convicted killer Kenneth Eugene Smith violently convulsed during his nitrogen gas execution for 4 painfully long minutes, and the founder of a leading euthanasia group says he knew it wouldn't go smoothly when he previewed the prison set-up. Dr. Philip Nitschke, founder of pro-euthanasia group "Exit International", spoke to "TMZ Live" Friday about the first-ever nitrogen gas execution ... saying he was concerned to see a gas mask set-up for Smith's Thursday night execution at the William C. Holman Correctional Facility in Atmore, Alabama.