It's okay. If you raise birds long enough eventually something like this will happen.
Look at it as a chance to start fresh. Maybe there's a specific breed or color you always wanted, now's the time to get it (well, after the fence is up).
If you ever see a loose dog, call animal control...
Dogs kill to play. They probably grabbed the chickens, shook them like a chew toy (dislocating/breaking their necks in the process) then spit them out and grabbed another one.
You have a fence around your property?