Having spent the better part of 2012 working to pass a measure to end death sentences in California, I probably learned to have more understanding of those who desperately want to see people who commit terrible crimes killed in their turn by the state. There are such things as revolting acts. Most of these criminals did things to other human being that properly turn our stomachs.
As Oklahoma has proved, again, it's hard to kill someone humanely. It may even be impossible. The eagerness of state authorities to do the deed, whether or not they possessed a viable set of procedures -- the drugs and the expertise to administer them -- seems to have led to their torturing at nasty perp named Clayton Lockett to death. Playing the Keystone cops with a man's body sure doesn't inspire confidence and certainly does inspire horror.
A current study in the Proceedings of the National Academy of Sciences [paywall] seeks to quantify how many innocent defendants are wrongly convicted and given the death penalty in the US. According to a a report on the study
Fortunately, in many states, prisoners sentenced to death receive good legal representation. What these authors suggest is that those sentenced to life fall out of that legal safety net; even if they are factually innocent, prisoners without money or family support are likely never to manage to prove it.
The Death Penalty Information Center currently lists 144 individuals who had their charges dismissed or won acquittals after death sentences since 1973.