Welcome to the BYC site. So sorry to hear about your chickens.
I agree that dogs pose perhaps the biggest problem for chickens. They are not afraid of human scent, like to chase anything that moves, will kill for fun rather than for food.
If you know whose dog did this, you may get some financial restitution. Not that that helps with the loss of a feathered friend.
If you don't know, then plan to build dog-proof coop and run areas before you get more chicks.
Love, Smart Red