There aren't any NO TRESPASSING signs

This could help you. Photograph the perimeter. Demonstrate the neighbor did not take necessary precaution to keeping people out.

we have never had any problems with this neighbor. He even had us watch a house for him a while back.

Previous contact will create a slippery slope for you both. It could go either way. Maybe there was a falling out between you guys? Maybe there is a feud going on? Maybe the neighbor thinks you stole something when you were house-sitting? It sounds strange he would say something hateful like that he hoped he killed the dog unless there was some kind of bad faith going on.

The dog breed is a Belgian mallinois mix

For everyone else, this is the breed of dog: https://en.wikipedia.org/wiki/Malinois_%28Belgian_Shepherd_Dog%29

This dog is big and the neighbor could claim he felt threatened by the dog, regardless of obedience training. I hate to give you bad news but the obedience stuff won't fly in court, because all opposing council will have to claim is that if the dog was as obedient as you claim, then why did it violate your command to return to your property and run towards the neighbor...

If it's dark and the dog rushed the neighbor, then questions are:

• did the neighbor know it was your dog?
• did the neighbor know you were in the line of fire?
• was the neighbor laying in wait for one of you to enter his property?
• to what extent was the neighbor correct in assuming his life was physically threatened?
• has the neighbor seen your dog before
• has the neighbor issued any kind of apology about killing your dog or anything in writing at all? Perhaps getting him on record outside of court could be useful... ask your lawyer.