I understand your feelings and am very sorry about the loss of your roo, but do agree with Mahonri, the dog was doing what they instinctively do.
Do you have a predator proof fenced area around your birds? Were they enclosed in this area and the dog broke in?
Is there a leash law where you are that the dog was violating?
Did you know who's dog it was, and inform them of what happened so they can do something about it?
If they don't do something about their dog, and have had fair warning then I can see you doing what you have to do, but if not and the chickens were just out everywhere with no protection as well was the dog... then it's no better killing their dog than the dog killing your roo.
Talk to the owner first please, then make your decision. Hopefully they'll pay for you to get another roo and keep the dog up so it stays out of trouble.
Sleep on it, pray about it and then act....
The saying two wrongs don't make a right fits too.
I'd hate to loose my beloved roo as well, but I'd also hate to think I shot some neighbor child's dog that was over here by mistake and knowing I didn't have my chickens enclosed to protect them. Accidents happen.
When you do something in haste, you almost always regret it. These are people you're going to have to live around. Good fences make good neighbors.
Just my 1 and a half cents.
...oh and