Even if your dog did it, there are other predators out there that still might and can. I don't know where you live, but stray dogs, cats, raccoons, skunks, hawks, owls, eagles, coyotes... you name it, they can and will kill chickens.
So first, you need a better pen for them to keep them safe. If it can keep your dog out, it'll sure keep out a lot of other things as well. An electric fence isn't a bad idea, either.
You can try a shock collar on your dog (we needed to use it on one of my mutts. My other dogs are guard dogs, so their instinct is to protect them, not eat them.) I needed to use a collar with a long remote, so I could correct my dog when he couldn't see me, because he learned to stop chasing chickens while I was home, but when I left, he'd do it again. So I had to hide inside the house or behid a tree to do it. It stopped him.
But training your own dog won't stop other predators. You can have dogs and chickens, but your chickens need protection, because they can't always protect themselves, or get to safety quickly enough.