Far from it son....far from it.
I am sorry but stuff like that(gruesome pics and the like, not that actual deed) dose not bother me. I can guarantee you that I have seen FAR worse than that, trust me. 4 years on a battle field will/can harden you up to things like this.
These things are a part of real life, some choose to hide from it and pretend it dose not exist, others do not.
Now with that, I DO NOT agree with how the OP handled this......
He to, could have taken the dog off and dropped it off somewhere els or ....
one shot....
If he would have taken it to a pound somewhere, they to, would have put it down. In this case, excessive force was used but to the same ending of either case.
Is shooting it 27 times a problem?
Is shooting it once a problem?
Is taking it to a pound to be put down a problem?
They all end with the same results....the dog being dead/killed.
Is that the problem?
Me, I would have driven to the other side of town and let it go......
but with that, the dog could have just as easily kill something/someone else again?
In any case, it is a loose-loose situation.