I didn't get the impression that Governor Shurtleff gave the go ahead out of religious conviction at all. The only religious element was a contract between the assumed mercy of God and the convict's apparent lack thereof.
When he writes, "May God grant him the mercy he denied his victims," the emphasis is on Garner's crime.
The "May God grant him the mercy..." part is more likely a cultural artifact than religious extremism.
I don't object to the governor's comments, which were made by the Governor as an individual.