The primary reasons I oppose the death penalty are that humans make mistakes, and execution is permanent.
To illustrate my point, I would like to bring two cases to your attention:
1. Raymond Towler was convicted of raping an 11 year old girl almost 30 years ago. His conviction was overturned with DNA testing, which was unavailable at the time. You can read about it here.
2. James Bain was convicted of raping and kidnapping a nine year old boy 35 years ago. His conviction was also overturned with DNA testing and he is now looking forward to "eating fried turkey and drinking Dr Pepper." You can read about his case here.
I expect that both men were viewed as the worst kind of criminals when they were tried and convicted. Had their sentences been execution, then James and Raymond would not see their families again, nor eat fried turkey.
To know these facts and argue for a death penalty anyway is to pay too much respect to violence, and too little to Justice.