I am so sorry for your loss.

I cannot imagine how very upsetting it must be to lose your precious chicks, especially ones of a breed you were so excited about.
Even dogs who are typically predictable or trustworthy can and will succumb to their natural instinct at times. His instincts are what got into his head. Though domesticated and trained, he is still a predator who is hardwired to chase down and kill small creatures that move. He may know that his owners generally don't want him going after their other animals, but he is a dog, and in a dog's world chasing and killing is perfectly acceptable, even desirable. Perhaps because the chicks were in a room in your house rather than out in a coop / run as he normally knows chickens to do, he viewed them as "different" and "part of his territory" or "something for him." Or maybe the fact that the brooder had no lid and he had direct access to the little balls of fluff was just too much for him to handle this time.
You will probably feel resentful toward him for a while, but I urge you not to treat him harshly. Unlike humans, dogs are very in-the-moment thinkers, which means if a person acts mad at or punishes a dog for something bad it did earlier, the dog will not connect the punishment to that past action. I know with my own puppy it is extremely hard not to continue being mad at her for some time after she does something I perceive as bad, but I know that being angry doesn't help her or me, and that she doesn't know that the thing she did was unacceptable in human society.

Please understand that I'm not saying I support your dog's actions in any way, or that I don't feel bad about what happened; I just believe it is helpful to analyze the situation from perspectives other than our immediate one as humans. Having empathy for the dog will not change what happened (and again, I am so sorry about what happened!), but it will help us realize and reflect on why he acted as he did.