Ben, I am so sorry for your heartache, and especially now that you describe how you dreaded going into the pens to feed, or to do chores.
Please, can you describe to us, what you saw that might possibly resemble some "early symptoms" of the deer possibly not acting right? Any clues at all? I am thinking, I'll hit my deer with some Dex ahead of time or possibly a longer-acting antibotic to help a potential respiratory infection. Is there anything that can be done preventatively? What do you think? What did your vet advise? anything?
Lana