They should have started pumping the nitrogen without telling him as the thrashing was likely from holding his breath. Given his anti death penalty final statements it is possible he tried to put on a show. There is no doubt he was encouraged to do so prior to the execution.
BINGO! we have a winner. Ive been part of many confined space teams. and worked with rescue squad members.
N2 is a silent killer. You pass out and crumple. no thrashing or panicking. you just DRT.. dead right there.