A couple of thoughts on this.
First, Yeah, as others said, Nathan Deal couldn't have done a thing. The legislature in GA has taken that power from the governor and handed it to a parole / pardons board.
Second, Tory Davis didn't shoot one man that night, he shot two. The evidence of the actual rounds were inconclusive, but the shell casings at both scenes matched. The first guy was shot in the face. The second, Mark MacPhail, was shot in the head and heart as Troy stood over him.
Third, there were 9 eyewitnesses, and yes, 7 of them did recant. But, they only recently recanted, they refused to be cross examined by any prosecutor, and they only seemed to recant when death penalty abolitionists contacted them and put pressure on them to "do the right thing."
All that aside, the state should not have the right to kill someone, even if they don't like him. Troy Davis was of no further risk in prison. For those that say he's in a better place, well, that may be true, but it may also be false. We don't know what lies after death, nobody has ever come back from an execution.