1.
An 8 year old dog is not a puppy. (age stated in the video in the article) It was a golden retriever, so it's a medium sized dog. I get that you don't like the police, but please at least get your facts straight.
2. If the dog did attack the officer then it's not 'no reason'. Given the size of the dog and the number of shots fired (seven, but with only two hits) it's likely it was a panic reaction, not a calculated one.
Here's the police report, btw -
http://ftpcontent.worldnow.com/wnem/WNEM/Police%20Report%20Dog%20Shooting.pdf3. If the officer's account is at all true, then he still made some poor decisions. Police officers are not equipped to deal with animals - too often we forget that dogs are animals and do not always react rationally. Once the dog was contained in the back yard of the residence this occurred at he should have made effort to notify the residents about the dog so they would stay inside or claim ownership and then call animal control if they were not the owners. At the very least he should have had his pepper spray in hand when approaching the dog as he could not be sure if the dog would be friendly to him or not. This poor decision making probably stems from lack of training and procedure in dealing with this kind of situation.
