There have been 1336 appeals for people wrongly convicted, and while most aren't in the capital crime bracket, which would have received a death sentence, it shows beyond a doubt that the legal system can get it very very wrong.
This of course is the number of successful appeals IN ONE YEAR.
An acquaintance of mine -
https://en.wikipedia.org/wiki/Robert_Brown_case
Had we the death penalty there is no doubt Rab would have received it.
"The three judges presiding over Brown's 2002 appeal heard evidence which, in their summing up, prompted them to describe the arresting officers in Brown's case to be part of a "culture of corruption and a conspiracy to pervert the course of justice" "
So had we the death penalty, those corrupt police officers would have condemned an innocent man to die. There is no doubt in my mind many convictions where police corruption has fabricated the evidence, has led a number of people to be hanged when we did have it.
There is also the higher profile cases like the Guildford 4, Birmingham 6 or the Maguire 7. How many of those do you think would have been executed ?.
All these cases were littered with allegations of torture at the hands of the police.
There can be little doubt that if capital punishment was legal at the time of their trials, these men would have been sentenced to death. Even Lord Denning – one of the most prominent judges of the 20th Century – suggested that the Birmingham Six should have been hanged. And if they had been hanged, they would have joined Derek Bentley and Timothy Evans, among countless others, on the list of innocent people that have been executed in this country. And this is by no means the only late-20th Century case of its kind. Just last month, Shahidul Ahmed was convicted for the murder of Rachel Manning. Ahmed is almost certainly guilty, but his conviction comes 11 years after Manning’s boyfriend was wrongfully convicted for the killing. Barri White not only suffered the loss of his girlfriend, but he also spent six years in prison after being wrongly accused and convicted of her murder.