The rooster would help some, but the real question is how did the chickens die?
Did the dog attack them for food, fun, or by accident in frolic.
If it was truly accidental, then the rooster will sort it out easier.
A swift peck and kick will tell the doggie these arent toys.
If the dog is following instincts, then a rooster might end up as a puff of feathers as well.
Only difference would be a couple of good pokes, maybe a little of the dogs own blood.
The only one with the power to sort out a dog attacking chickens is the owner of the dog. Not quick or easy, but possible.