Animals are a creation of God, and thus ought to be treated with respect and dignity, just the same with the planet (re: global warming). I do believe it was a mortal sin, he willfully killed those dogs because they weren’t fighting well, the killing of the dogs was a wrong act, and baring any mental illness, he fully knew this.
That being said, I am getting sick and tired of people treating animals with greater respect then humans. Animal-rights groups cry and whine over bears getting shot (even after said bears have mauled people) and other things, but when it comes to fellow human beings, it seems like they care less, and when it comes between humans and animals, I really couldn’t care less about animals.