Yes, the officer weighed all the facts before shooting the dog. The death of the dog is unfortunate, but the officer had cause to believe that the dog would have seriously injured or even killed him. While the officer was wrong to enter the property without a warrant, he had no choice but to defend himself from the dog once he was there. The officer had to act in the moment and deal with the threat. Some dog breeds are aggressive, with a body weight that can knock down an adult male and a bite that can break bones and maul skin. Such dogs defend their territory instinctively, and without a net or animal control professional, it is impossible to neutralize them without violence. Considering that this dog posed an immediate threat to the officer, neutralizing the danger by shooting the dog was an appropriate course of action.
The officer in this incident should have used non-deadly force to restrain the dog if it posed a threat. Furthermore, the fact that the warrant was not served, and ended up not even being for a person that lived in the home, is heartbreaking for the family that lost their pet. Unfortunately, the officer may have misjudged the dog's actions and shot it unnecessarily.
When attempting to serve a person at their home, unless it is absolutely necessary, the police should never harm an animal. The death of the person's dog is a tragedy that could have been easily avoided. The person will leave their home eventually, the task could have waited until then, instead of harming an animal.
Having read the news about this, I can draw the conclusion the officer was incorrect to shoot the dog at the birthday party. Officer was outside the fence. Dog was inside. Person whom the warrant was for had been gone for a long time and was not confirmed to be using the address in question.