As imperfect beings, we will ALWAYS make mistakes within our justice system. If an imprisoned man is later found innocent, we can free him. But if an innocent man is executed, we cannot bring him back to life. I do not believe that an inherently imperfect justice system should be allowed to hand out such an irreversible punishment.
And it isn't just the innocent. Blacks and Hispanics are far more likely to receive the death penalty for the same crime than their white counterparts. It might be due to racism. It also might be that there are more affluent white people than affluent blacks and Hispanics, so more white people are able to afford good defense lawyers who are far better at negotiating plea deals than their public defender counterparts. Should the criteria for being executed vs. being sentenced to life in prison be how much money you have in your bank account, or what color your skin is?