Miscarriages of justice happen regardless of the system held to. Errors that do occur are more properly understood as procedural not conceptual.

Save for pacifist positions killing is considered appropriate as a societal defense. This is the justification of having a military. The basis for the Death Penalty is retribution. If one holds that there are acts that are so egregious that the offender can no longer be part of society then the guilty should be killed. Life imprisonment is cruel and unusual punishment.