Our 4 year old dog just attacked a 16 year old female dog which had to be put to sleep because it was injured so badly. What can I do?

German Shepherd is 4 years old, male. We had him from 3 months. Household is wife (59), daughter (27) and me (62). We live on a 4-acre lot and use invisible fencing. We also have a 9 by 12 ft holding area -- chain link fence variety of good quality. The dog Zeus, was supposed to be my daughter's. She took him to obedience training a few times and then dropped out. Said that he didn't need it. There are 2 cats in the house. We are grandparents and grand kids visit. They use the trampoline and swing - outdoor playground equipment. Zeus sees the pack hierarchy as me (lead) then daughter, then wife. That's the background.

Zeus attacked and severely damaged a 16 year old female dog that came on our property Saturday April 28. They were friends and had been playing together for several years. The dog usually visited. We learned that day from our neighbors that Zeus also visited the dog whenever we failed to put the collar on him (which was not often).

No one saw the attack (other than our next door neighbor who noticed from a good distance - this neighbor was not the dog owner). My daughter saw the dog in the backyard with Zeus circling and licking her. We ourselves were not home. We did not see the damage. Our neighbor said that Zeus would let the other dog get up and would grab its hind leg. The older dog was on its back.

The owners took their dog to emergency and she was put down. said that she wouldn't survive the operation and that there were too many holes. Everyone was surprised at this event. None of us can explain what and why it happened. The damage to the dog was many punctures to the body and a mangled rear leg.

My daughter dirt-bikes and Zeus just can't control himself whenever he hears the bike motor start up. He also wants to race the bike as she leaves (riding) for the dirt-bike track. When this occurs no one can control him.

At Easter Zeus got really excited with the kids on the swings. I had just erected the set so it was new. Cindy, our daughter expressed concern that she couldn't handle Zeus and Zeus (through excitement) has nipped her son in the leg. There was no mark on the leg. I took Zeus to the swings, and had another grandson sit and corrected Zeus using a rolled-up newspaper. Zeus was cowed down and suddenly surprised me! He showed his teeth had a direct stare and growled - still cowed. I stopped then and resumed right after without further challenge from Zeus. My daughter had complained to me previously that he had challenged her.

My wife, as you probably gathered doesn't have much to do with Zeus.

When anyone comes to the yard, workmen, strangers, family, Zeus looks for a stick - there is usually some broken twig around from the trees and invites them to play.

I took Zeus to the vet last night to have him neutered. I have asked the family to place him in his pen whenever the grand kids are around. Also to keep the collar on him whenever he is outside. We still give him the full run of the yard otherwise. Most times he is alone.

Any suggestions you can offer will be much appreciated. I am concerned from the attack and the challenges to my daughter and I.

Just can't figure out why he would attack an old female playmate!

Joe