Unfortunately, these rules have been broken quite often in death penalty cases. Especially in the early 20th century, minority suspects were often given incompetent defenses, with trials presided over by hostile judges and juries. Children have been executed, along with suspects whose mental capacity left them unfit to stand trial. And many people have gone to their deaths proclaiming their innocence - only to be proven right decades later.
Here are some of the most controversial executions and death penalty cases in US history.
Sacco and Vanzetti
Italian immigrants Ferdinando Nicola Sacco and Bartolomeo Vanzetti were executed in 1927 after a highly contested series of trials over the shooting death of a security guard and paymaster during a 1920 armed robbery.
The trials are now seen to be highly flawed, featuring a poor defense, conflicting witnesses speaking in thick Italian accents, and evidence of the jury tampering with evidence. The men were followers of Italian anarchist Luigi Galleani, and anti-Italian sentiment almost certainly played a role in their execution.
After their convictions, the accused men waged a six-year legal battle that went all the way to the Supreme Court twice, with numerous public figures calling for new trials. Another man even confessed to the shootings, ex-convict Celestino Madeiros, but the die was cast, and the two men died in the electric chair on August 23, 1927. Historians still debate whether either man really pulled the trigger.
Despite an almost complete lack of evidence, Stinney was found guilty for the rape and murder of two pre-teen girls in 1944. The only evidence against Stinney at all was that he'd spoken with the two girls at some point before their murder. An all-white jury deliberated for ten minutes, after a trial that took just a day. At 14, Stinney was the youngest person executed in the US in the 20th century - with less than three months passing between the murders and his execution.
In 2013, a local historian who had been looking into the case persuaded a South Carolina circuit court to reopen the case. Stinney was given a new trial and exonerated, because the boy had a defense attorney who took no action on his behalf, and had been beaten into confessing.
see more on George Stinney
The Scottsboro Boys
Even 80 years later, the phrase "Scottsboro Boys" is shorthand for racially based death sentencing.
Based on the judgment of all-white juries, nine black teenage boys were found guilty of raping two white women on a freight train in 1931. Eight were sentenced to death, with the final one being just 12 years old and deemed too young for the electric chair. The proceedings were a farce from the beginning, with all nine trials being handled in just one day, a group of defense attorneys totally unsuited to criminal defense, and a lynch mob demanding the surrender of the teenagers outside the court.
The convictions led to demonstrations around the country, and the case eventually made it to the Supreme Court. The convictions were reversed because of the lack of an adequate defense, with charges beingdropped against four of the men. After numerous appeals, retrials, and reversals, none of the men were executed, though many served long prison terms - all based on conflicting evidence.
Chessman became a key figure in anti-death-penalty sentiment in the 1950s, after being convicted of robbery, kidnapping and rape in 1948. The jury determined that Chessman, who had become known as the "Red Light Bandit," had caused bodily harm during one of the kidnappings, making him eligible for death - despite none of his victims actually dying.
From death row, Chessman wrote books maintaining his innocence and insisting that his original confession had been coerced through torture. His trial became a media circus, with Chessman defending himself, and a botched transcript that was noticeably edited after the death of the court stenographer. There was widespread outrage over the case, and Chessman had numerous public figures clamoring for a new trial, including former first lady Eleanor Roosevelt, writer Ray Bradbury and poet Robert Frost.
On May 2, 1960, Chessman finally went to the gas chamber at San Quentin Prison - just as a legal secretary called with news that a federal judge had issued one more stay of execution. But by the time the news reached prison officials, Chessman was dead.