thats one of the inherent problems with the death penalty is if you believe their could be a valid argument that mistakes or planting of evidence by the police is possible then you really cant be for the death penalty.
you would have to have complete faith in the entire system from the reporting of the crime and what was observed, to the time of arrest, collection of evidence, witness statements, so-called confessions, physical evidence, all reports by police officer or detectives applying for search and arrest warrants for which came from credible sources, the list goes on....
you cant take death back. if its found out at a later date that evidence was planted or mistakes were made either by accident or made to look like accidents you cant get a new trial.
the law is a very grey area and up to interpretation from one court or state to another pertaining to murder and what defines murder and what type of deadly force was appropriate. remember there are almost 150 ppl that were on death row and are no longer even in prison and received large settlements for wrongful prosecution and false imprisonment and arrest lawsuits.
your either for it 100% or your against it.