I will admit that I didn't read every thread so I might have missed some things, but your dog is becoming protective, which is what an LGD does around the age of 2. Killing the cats and dogs on his land is common, he is protecting what is his and these intruding animals are not part of his flock or family. Killing the chickens is most likely a matter of training and boredom, but also remember he is part bird dog. So he has mixed traits, chase the birds or protect the birds. He may do very well on a farm with large animals, but with the lab in him he will most likely always have a higher prey drive. That being said, killing animals is a far cry from hurting humans, and yes, LGDs are often edgy with guests, but they will protect their family with their life.
LGDs are unique dogs in how they think and how they act but there are usually loyal to a fault.
I understand your concern, and humans do come first, especially kids, but I don't see his behavior as anything out of the ordinary for a dog that is a mix of Pyr and Lab. Unfortunately he may be a hard dog to rehome as he doesn't quite fit in as a farm guardian or as a stock guardian. You have a hard decision to make, and for that I am sorry.