I love animals (especially dogs, but now that my family has that cat, I slowly soften) and I think this was absolutely the best thing to do.
As you are saying, there is the evolution aspect, but also the fact that he cut short a possible string of serious problems.
This is also the reason I am for euthanasia of animals in shelters past a month (the time left for the owners to find a dog that escaped). Keeping them in cages is torture for the animals.
It is not that they realize they will die, if this is done correctly.
This is also why I am against zoos and circuses with animals.
I think we went into some kind of multiple personality spilt when it comes to animals : we eat them after they have suffered (I eat meat myself, but I would prefer to pay more for meat from animals that were slaughtered in a human way), we do not want to stop the suffering of caged animals, we keep large dogs in small apartments, we cry when we see a lion eating a gazela in the wild, etc.