Although I disagree with the death penalty on principle (I don't want to live in a society that's moral compass hasn't become more in over 2,000 years beyond an eye for an eye and a tooth for a tooth), I also object on practical grounds.
The criminal justice system will always be too fallible - it's run by humans. There's been lots of examples over the years, some of which have led to progress and reform, but it will never be perfect.
From vague and distant undergrad memories Kiszko had learning needs and was bullied into confessing - Police and Criminal Evidence Act (governing things like right to have a lawyer, right to be accompanied at police interview for vulnerable adults and children) only came in to force in 1984. Then you have forensic science slip-ups (factory soap of Guildford 4 being the cause behind the glycerine on their hands, as opposed to due to bomb-making) and police under pressure to produce results. And inquiries such as the Steven lawrence Enquiry. I am very afraid we are heading down the route to having elected judges which would push our justice system closer to the States and is not good (in my view) for ensuring a robust checks and balances
I think it also has the effect of making juries less able to convict so would push conviction rates even lower, meaning we have more dangerous people in society who are repeat offenders.