Convicted killer Kenneth Eugene Smithwas executed Thursday by the state of Alabama using a new deadly procedure, as his wife and pastor looked on in absolute horror at his excruciatingly painful reaction.

Smith was ushered inside the execution chamber at the William C. Holman Correctional Facility in Atmore, where prison staff put him in restraints and fitted a gas mask over his face to suffocate him with nitrogen hypoxia. He became the first ever inmate to be put to death utilizing this method.

Before the execution began at 8:07 PM CST, Smith made a heart sign with his left hand to his family who was present before saying his final words through his mask: “Tonight Alabama causes humanity to take a step backwards… I’m leaving with love, peace, and light.”

The next 18 minutes turned into a nightmare for Smith and his two closest confidantes — namely his wife, Deanna, and his spiritual guide, Jeff Hood, both of whom were in the witness box.

Hood later told the Daily Mail … the execution was the “worst thing” he had ever seen, even causing prison officials to gasp while Smith died much slower than anticipated.

The pastor also said Smith started convulsing with his body popping up on the gurney after correctional officers pumped his mask full of nitrogen hypoxia, forcing him to inhale high concentrations of the gas.

At one point, Deanna cried out for her dying husband as he grabbed at his restraints, taking his last breaths before he suffocated. Smith was sentenced to death in 1996 for killing a preacher’s wife, Elizabeth Sennett, as part of a $1,000 murder-for-hire plot.

He was pronounced dead at 8:25 PM .

Smith was 58.