Certainly not dispositive, but a second execution in Texas has been called into question. To be fair, this guy is not a saint, but he could not have been put to death under Texas law because the only other evidence that he was the murderer was accomplice testimony.
The more damning case is, of course, the execution of Cameron Todd Willingham. If you haven’t read the New Yorker article, you really should.
I don’t support the death penalty for a variety of vaguely outlined moral reasons. (Admittedly, I’m willing to entertain exceptions, such as the capture of Hitler, etc.) But my biggest issue is that I don’t trust the legal system enough to give the government the ultimate power, the right to kill individuals.