[GladesGuru]

If the dog was unable to see the hammer because the owner covered its head, a massive blow to the cranium with a hammer would result in instant loss of consciousness.

Even at a vet’s, killing of an animal is not a totally painless, fear free process all of the time.

[messierhunter]

Even at a vet’s, killing of an animal is not a totally painless, fear free process all of the time.

As someone who euthanatizes animals on a fairly regular basis, there's no comparison between the first attempt success rate of a drug-based euthanasia and a hand-delivered blow to the head. Assuming this owner had never killed a dog with a hammer before, I'm confident he would not be as careful as you describe, let alone sure enough with his first blow to render the animal completely unconscious. He's free to plead his case before a judge though.