[user104658]

He didn't "kill" it as that makes it sound like he deliberately harmed it somehow. However, there is a good chance that the puppy died because of negligence and incompetence.

Whoever sold it to him so young shares part of the blame too, it should still have been with it's mother, being taken away from doggy breast milk too early will have been a huge contributing factor in the poor thing's immune system being too weak to fight off infection.