I admit I'm a booger-eating newbie, including to country living, chickens, and to this board, but here is what I think, FWIW:
1)
KEEP THAT GUN IN THE CABINET! Violence and weapons never make situations better. They are for dire emergencies, when there's no other choice. The attack is over. Now it's time to use your words and your thoughts. You're upset, and angry (and rightfully so) which is a bad time for weapons of any type. The dire emergency is passed, and is now become a dispute. Good news is you have the high ground in this dispute. You instantly lose that high ground when you pull a trigger.
2)
Put out the fire. Get the remaining bird somewhere safe from the dog.
3)
Calmly and respectfully confront the other party. Think carefully about what you're going to say. Anticipate their defenses, and prepare responses. Make a list of exactly what you think you are owed to make things right. Think about possible win-win solutions. But be prepared to walk away, and go to court/the authorities if needed. If voices raise, walk away immediately. If you cannot use words, send certified letters, even if they're next door. Your actions are provable and in writing. Think of it as an opportunity to set an example to the young ones in your life on how to settle things well.
4)
Don't give up. You obviously loved your birds. I submit that you owe it to yourself to rebuild and carry on. It sounds like you are in the later part of your life, please don't let despair and fear ruin what should be the most rewarding phase of your life.
5)
Rebuild. Think carefully and critically about what went wrong with your coop/run design. For my part I was advised to fasten the HC on the
outside of the frame (pressure treated 2x4s) so as to force animals to push against the fasteners rather than pull them out.
Tractor Supply sells thick-wired hog panels (they're pricey) you could fasten to the outside to augment the HC, believe me
nothing is getting through
that stuff.
I hope I've helped, and I wish you all the best.
Regards,
--Kerry