Dog Gone Capital Punishment

I was watching the news the other night as they relayed the story of a Rottweiler who killed a little girl over a cookie that had fallen to the ground. As I'm sure all would expect, the dog was put down.

And that got me to thinking about Capital Punishment. In the States where Capital Punishment is legal, a person who is retarded cannot be put to death for the crimes they commit. I think the argument goes something like, the person doesn't fully understand what they did and / or wouldn't understand the punishment they receive.

Can you detect the logical inconstancy here? Transferring the logic of sparing the life of a mentally retarded human to the dog, why was the dog put to death? Did the dog understand what he was doing? Does the dog understand why he's on a cold table with a needle in his paw? Something tells me the dog knows EVEN LESS than a possibly retarded human so should his life not be spared? Or should it be that we are not so quick to grant reprieve to the mentally retarded?