Trev. I could list you dozens and dozens of death penalty cases in the USA that have been overturned after the convicted person was proven innocent due to corupt evidence and officials hiding evidence .
In the 40's & 50's in Texas there was two sheriffs famous for their arrest and conviction rate, over 50 of murderers and rapists a vast majority by confession of the crime.
After these men had been sentanced to death they usually retracted their confession saying they were torchered by the two sheriffs who said they would kill the accused family or kill the accused and said they tried to escape, if they didn't sign. This was obviously ignored by the state and pardons board in Texas and in nearly 100% of the death sentances were carried out.
After a conviction they obtained was looked at more closely in the 70s, after both sheriffs had died witness came forward to say they had seen how the sheriffs had got their confessions but were to affraid to come forward why they both were alive.
So it seems maybe 50 innocent men could have been executed by Texas just because of two corrupt officials. A terrible thing if true.
The USA is a difficult country to look at when discussing the death penalty as you have 30 odd states who still execute who all have different laws and sentances.
If you look at UK there has been only 3 cases of persons executed who have been overturned all of which would not have been convicted today.
Look at James Hanratty hung in the early 60s. His family and many others thought him innocent and fought for DNA evidence to prove him innocent. Some 45 yrs after he was hung his body was exhumed and samples taken. Far from proving him innocent the evidence proved he committed the crime.