Justification for killing a puppy? What could there possibly be? Besides putting it out of it's own misery, because due to humans, it was born horribly disfigured.
Pit bull's are just like any other dog. The aggressiveness is due to the owners. Secondly why would you leave an unsupervised child around an aggressive pit bull. Sounds like the herd needs to be culled anyways.
Are you ready for the river Kazilla? It will only be cold for a minute.

Animals do not "sin". They are following the evolutionary path that got us as far as we have come, but have been straying from for so long.