Really? I'd imagine he can fire 2 shots in under a second...
Think about what you're saying and what is involved in what is involved in safely firing that shotgun.
Let's walk through both options the officer had, shooting the dog or shooting a warning shot then trying to shoot the dog. The officer already had his gun aimed at the closest/most aggressive dog in case he had to use it.
Case #1
1. Gun already aimed at dog, knows everyone else is clear
2. Pulls trigger
3. Dog is stopped
Elapsed time: split second
Case #2
1. Gun already aimed at dog, knows everyone else is clear
2. Turns aim away from dog
3. Needs to make sure that everyone else and the dogs are clear of the new spot he is firing and that it is not a dangerous place to shoot (shooting cement can allow things to ricochet and be dangerous to other people)
4. Pulls trigger
5. Checks to see if dogs react and back away
6. Reaims on dog
7. Checks to see if everyone is clear
8. Pulls trigger
9. Dog is stopped
Elapsed time: realistically, multiple seconds
In all those extra steps the dog could have gotten latched on to the animal control guy, at best injuring him and at worst killing him. The officer had a clear shot, he took it, and he ended the situation. It is better to end a dog's life in that situation than to risk the health and life of a person.