The wife of Kenneth Eugene Smith cried out for her killer husband as he thrashed against his restraints, suffering as he became the first person in history to be executed via nitrogen hypoxia. 

Smith, 58, was put to death Thursday in a 'horrific' 22-minute ordeal when a gas mask was placed over his face, before a stream of 100 percent nitrogen gas suffocated him inside the execution chamber at the William C. Holman prison in Atmore, Alabama. 

The murderer's spiritual guide, Jeff Hood, described the execution as the 'worst thing' he had ever seen, claiming prison officials gasped in shock as he died far slower than anticipated.

'When they turned the nitrogen on, he began to convulse, he popped up on the gurney over and over again, he shook the whole gurney,' Hood said. 

Smith was officially pronounced dead at 8:25pm local time, 22 minutes after the nitrogen gas was first administered. At 8:07pm, a corrections officer leaned over Smith and examined him, before stepping back against the wall to let the execution continue. 

At one point as the killer tore against his restraints, his wife Deanna, wearing a T-shirt reading 'Never Alone', desperately cried out for him from the witness box.

As he spoke his final words, delivered through the gas mask on his face, Smith made a heart sign with his left hand to his family before declaring: 'Tonight Alabama causes humanity to take a step backwards... I'm leaving with love, peace and light.'

He was sentenced to death in 1996 for the murder-for-hire slaying of a preacher's wife in 1988, where he was paid just $1,000 for the hit. 

Smith's pastor John Ewell told DailyMail.com before his execution that the killer was 'really struggling' with the reality of his imminent death, and officials said he barely touched his final meal of a T-bone steak, hashbrowns, eggs and A1 Sauce from Waffle House.