Stokley, 60, was put to death by lethal injection at the Arizona State Prison complex in Florence after executioners had to cut into a vein in his leg so they could set two intravenous lines, The Arizona Republic reported.
Witnesses watching the procedure on closed-circuit TV said they heard Stokley tell the execution team: "I do wish I could die doing something meaningful. You know, this seems such a waste."
Stokley offered no formal last words and did not look at witnesses, the newspaper said.
Patty Hancock, the mother Mandy Meyers, said Stokley "was a coward from the beginning and he died a coward."
Hancock was a cook in a restaurant operated by Glenn Brazael, whose son Randy was convicted along with Stokley in the case. Randy Brazeal, 41, entered into a plea bargain on two counts of second-degree murder and received two concurrent, 20-year sentences.
He was released last year.
"It could have been Brazeal on that [executioner's] table and not him," Hancock said.
Evidence showed Stokley and Brazeal raped, strangled, stabbed and stomped on their victims, both 13, before tossing their bodies into a flooded mine shaft.
Hancock has said everything Brazeal told investigators "was a lie."
"To this day, they have more evidence on Randy Brazeal than they ever had on Richard Stokley," she said.