Dogs killing chickens is very common since the dog usually sees them as prey animals. I don't know where in Oregon you live but I do know there are many places where pitbulls and mixes are not discriminated against. While the AC officer was probably correct that the dog would always be a chicken killer, saying he was always going to be "aggressive" is misleading and wrong. I'm sorry your dog had to give his life in this situation. In the end, YOU are responsible for not containing him properly. Leaving any athletic and active dog loose in a yard is just asking for them to get out either by digging or climbing the fence (6ft is nothing to most pitbulls). Learn from this and make sure you keep your dog contained in the future.