A pitbull is a terrier, and terriers, by nature, go after small animals. That is what they were bred to do, most of the rodents and such.
It is true that pitbulls are not human aggressive--just because a dog goes after animals does not mean it will go after a human.
This dog can be taught to become a good dog again. But it must be done quickly and with complete commitment. The dog is doing what it thinks it is supposed to do--go after small animals, it was bred to do that.
Think of it this way--the dog is not "being aggressive" he is "working". Dogs need a job. They love having a job. Especiallly pitbulls who are very loyal and extrememly eager to please. His energy needs to be redirected into something else. He needs another job. He needs to realize that killing chickens is not his job and does not make his owner happy--there is something else that does. Agility, running, jumping, learning tricks, anything other than livestock.
I am very sorry about the loss of the livestock and the dog--what a horrible thing. But if they love this dog, they can turn him around. Some of Michael Vick's trained fighting dogs are now kind, gentle therapy dogs. It can happen.
While he is being trained, do not let him anywhere around the livestock for a while--pitbulls are strong, make sure leashes and enclosures are very secure.
Yes, this dog can be saved. But if they do not have the desire or ability to work with him every day for much of the day, then it would be better to rehome him.