This is bad. Unfortunately if this dog has killed your chooks & wildlife he is going to keep doing it. They are going to need to go to some major measures to prevent the dog doing it.
If you can, call up your local council & ask for advise about fencing etc & then talk to them face to face. If its only a young dog then its only going to get bigger & more skilled at what its doing. Friendly, nice dog or not.. Its a killer.

I own a boxer & she is lovely. BUT I would not trust her free range chooks (and I really want chooks!) She kills snakes.

In saying that though, just because this dog kills wildlife/chooks. I don't think there is concern for it attacking people (not that anyone has said that but I know a lot of people often link the two & its simply not the case)