In April 2014 an execution at the Oklahoma State Penitentiary in McAlester went badly wrong.

Convicted murderer Clayton Lockett gasped and writhed in pain for more than 40 minutes after officials incorrectly placed the intravenous line delivering lethal drugs into his body.

Lockett eventually succumbed to a heart attack as prison staff tried to re-insert the tubes into his veins. The prison warden called it "a bloody mess".

In the aftermath of that fiasco, the state suspended executions, retrained prison staff, and spent more than $70,000 to remodel the death chamber, adding more advanced technology.

Now Oklahoma is preparing to execute Charles Warner on Friday. He was convicted of the rape and murder of an 11-month-old girl.

Three quarters of Oklahoma voters are in favour of capital punishment, as are their elected representatives, among them State Senator Ralph Shortey.

"At the end of the day, the people expect us to hand out justice in the manner that they have prescribed," Shortey told Al Jazeera. "As a government we are under an obligation to do that."

States like Oklahoma have found it increasingly difficult to obtain the drugs required for lethal injections.

"Gradually makers of pharmaceuticals have begun to clamp down more and more of the uses of their products which are supposed to be therapeutic in a means which is designed to kill. And that has created supply problems," says Brady Henderson, Legal Director of the Oklahoma Chapter of the American Civil Liberties Union.

Shortey plans to introduce legislation to give Oklahoma’s executioners more options than lethal injection, including death by firing squad or the electric chair. He says none of those methods would inflict unnecessary pain. He personally favours killing condemned convicts with nitrogen gas, he added.

"Whenever you increase the nitrogen level by decreasing the oxygen level on a person they black out they lose consciousness. It's what some have said is a euphoric type of death. Basically you pass out, go to sleep and you just don’t wake up."

The debate over execution methods comes as US public attitudes toward capital punishment are changing. Nationwide the number of executions has been in decline.

In 2014, 35 convicted criminals were put to death, the lowest number in 20 years. Last year seven condemned men were exonerated and freed from death row.

Recent polls show that, given a choice between life in prison and execution for convicted murderers, 52 percent of Americans favour life imprisonment without parole. When not offered a choice, 61 percent say they support the death penalty in general terms. That's a big shift from 20 years ago, when 80 percent supported execution.

Thirteen US states have abolished capital punishment, most recently Maryland in 2013. Other states, like California (which has the largest number of prisoners on death row), have not carried out any executions for many years.

"We are seeing a decline in the number of death sentences, a decline in the number of executions, and a decline in public support for the practice," said Diann Rust Tierney, executive director of the National Coalition to Abolish the Death Penalty. She also believes that botched executions shape public opinion.

"When we have a botched execution like the one we saw in Oklahoma the public is jolted and they have to focus on it," Rust-Tierney says, "and when they focus on it, they see it is really a mess."

Barring a last minute stay from the US Supreme Court, Charles Warner will be the first person, but likely not the last, to die in Oklahoma's new and improved death chamber.