Well, look at it this way. A group of armed to the teeth professionals (?) attacks a private residence, kicks the door in and then shoots the owher who is minding his own business inside.
Note that it is perfectly legal (safe or not is a different story) for any owner of a rifle to work on it, clean it, hug it, or do whatever he wants with it within his own residence (or, actually, in many other places that are considered private property, like your campground.) The guy, for all intents and purposes, could have set up a tripod and had a loaded rifle aimed at the entry door. Legal. Safe? That's another question; but it is legal, AFAIK.
So when a group of invaders breaks the door down and sees the setup and the owner, what right do they have to kill him? We don't even need to go into the self-defense area. The homeowner is allowed to handle firearms inside his house, and if you break in while he is doing it ... too bad, you should have used the doorbell. That's one of 217 reasons why people should be secure in their homes - to let them do things there that wouldn't be quite proper to do among general public.
So yes, the attackers are probably somewhat sad about what happened. They acted as burglars with murder in mind; as clumsy ones too, if earlier reports are correct - the front LEO had a negligent discharge (itchy trigger finger?) and the rest of them opened up blindly, without identifying the threat.
Why would they be sad? They got to accomplish what their intense training prepared them for and promised them. They eliminated one of the monsters on that Level. After successfully eliminating a few more they get to the next Level. It is a video game without the possibility of the player suffering his own atavar’s demise and failure on a Level.