I am against the death penalty. First of all it is not justice but down to the mood of the jury whether or not gets the death penalty. Secondly it costs a lot more than jailing him/her for life. Thirdly, you cannot un-kill an innocent person but when sentenced to life you can release them.
Lastly, to some degree the death penalty is based on the quality of the police investigation (or sometimes the lack of it), the quality of the prosecutor (or his hunger sometimes to get a leg up in a future political career), the quality of the defense lawyer (which for rich people/famous cases is a lot better than for Joe Public), the quality of the judge and the deals that were made to get some of the "witnesses" to testify.
I also have a moral issue with the death penalty. While it may be horrible punishment for a relatively short period of time and then the punishment for the prisoner is over. He is free from his mortal existence and as I do not believe in heaven or hell, he has gotten off lightly IMHO. I want people who have committed the worst crimes (mass murder or repeat murderers) to sit in jail for the rest of their lives. They have to be kept in jail until they die of the natural cause of old age, making their punishment as long as is humanly possible.