The number of innocent killed with it is undoubtedly FAR higher than is suspected and certainly far higher than almost anyone wants.
One reason I say that is a recent article I saw saying that in a recent 30 yr period of the death sentences given, 117 were found innocent, and they felt nearly 3 times that number might be innocent but couldn't be proven.
Regardless, and because I'm a little skeptical of justice anyway, I am more against it than for it. I'm aware of so many reasons the innocent are convicted, to include a lot of innate human psychological frailty.
A good article on subject:
http://www.cnn.com/2013/12/04/justice/prisoner-exonerations-facts-innocence-project/