I used to believe in the death penalty but here's what changed my mind. The justice system has way too many limitations. Way too many people have been wrong accused of crimes and later found to be innocent. The innocence project lists 200 people that have now been exonerated through DNA evidence. These are for major crimes. The most recent of which was Jerry Miller who was accused of a brutal rape in 1982. He served 24 years in prison for a crime he did not commit.
http://www.innocenceproject.org/Content/530.php
Consider all the experiment demonstrating the limitations of eye-witness testimony
and yet eyewitness testimony is used to convict people of crime.
There's a great experiment conducted by Daniel Simons of the University of Illinois at Urbana-Champaign and Christopher Chabris of Harvard University. Their study, a contemporized version of earlier studies conducted by Ulric Neisser, asked subjects to watch a short video in which two groups of people (wearing black and white t-shirts) pass a basketball around. The subjects are told to either count the number of passes made by one of the teams or to keep count of bounce passes vs. aerial passes. In different versions of the video a man walks through the scene carrying an umbrella, or wearing a full gorilla suit. After watching the video the subjects are asked if they saw anything out of the ordinary take place. In most groups, 50% of the subjects did not report seeing the gorilla.
Here's a picture from the actual experiment.
The amusing thing is if someone was asked in this situation to be an eyewitness they would exclaim "Not only was I there and saw no gorilla. I was really focusing because I was counting the number of times my friends were passing a basketball back and forth - so there's no way i could miss it! I'm a great witness!"