There is a difference between a single clean blow with a sledgehammer and beating the animal to death.
If you read the article, the animal sustained “several” blows to the head. To me, that sounds like an unskilled person was doing his best to stun the animal with the hammer.
I dunno. If you want to assume this guy is an inhumane monster, I can do nothing to convince you otherwise. But his actions and the evidence in the story is consistent with somebody who is trying to do his best in a difficult situation.
Ask yourself this question... If he was intent on savagely and criminally torturing this animal, would he have called the cops before he got started?