There's plenty of cases where the prosecution either fucked up, didn't get all evidence possible, or just had plain old tunnel vision for me to ever support the death penalty. Humans are fallible, including well educated lawyers, judges, police, prosecutors etc and we should never ever assume any institution is free from making mistakes or bias.
With the death penalty, it is utterly irreversible. At least with even the most punitive life sentences, you know that if there is a possibility of innocence, it may well be realised over time and effort and a person may one day be freed and the miscarriage of justice revealed.
Speaking more broadly, while I understand the very human desire for revenge, the desire to see someone suffer for the evils they committed, I feel that the job of the prison service should be rehabilitation*, not punishment or revenge. Plenty of studies show that the potential penalty for crimes do not prevent crime, because most people are not good at evaluating risk, and will assume they will never be caught - because who sets out to commit crimes believing they will be caught for it?
The best way to prevent crimes is to ensure circumstances are in place to prevent it from occurring in the first place. A poor/abusive childhood, poverty, abuse, drugs: all reasons people commit crimes. All are possible to fix, but require significant society rethinking and reallocation of resources. Institutions reflect the mindset of the people it serves. While the mood of society is towards punishment rather than rehabilitation and prevention, then crimes will continue to occur because the root cause of crime has not been fixed.
*However I do think some people are too dangerous or too far down a path of their making to change, and keeping them locked away for the safety of society may be the only option, I would count within this category: serial killers, repeat rapists, individuals who are not amenable to reform - essentially the current crimes for which one receives a life sentence should still receive this, everyone else should be helped to return to society, helped to find a job, and supported until they are ready to independently support themselves and their family again.