He just fell asleep.

eyewitness accounts disagree

"Smith’s spiritual adviser, the Rev. Jeff Hood, who’d previously expressed concern that the method could be inhumane, witnessed the execution and described it in more graphic terms, saying it was ‘the most horrible thing I’ve ever seen.’

Smith, wearing a tight-fitting mask that covered his entire face, convulsed when the gas was turned on, ‘popped up on the gurney’ repeatedly, and gasped, heaved and spat, Hood said.

‘It was absolutely horrific,’ he said."

“Smith, who was on a gurney, appeared conscious for “several minutes into the execution,” and “shook and writhed” for about two minutes after that, media witnesses said in a joint report.”

https://www.cnn.com/2024/01/26/us/alabama-execution-nitrogen-what-we-know/index.html