The room was full of bones, the remains pulled from four huge mass graves. The previous grave robbers had taken anything of value, leaving bones, rusting weapons, and rusting armor.
"Let's take all the left femurs,"
"That would work, but it would take hours. There are hundreds. The tomb raiders left all the bones, but took most of the weapons and armor."
"There are weapons in there too?"
"You see a few dozen scimitars and other blades."
"We take them and pile them up outside the room. That shouldn't take long."
"No, maybe five, ten minutes tops."
When the negative energy burst came and the skeletons rose, our heroes were ready for them. And the skeletons were unarmed!
"The skeleton attacks, with its claws. Because it doesn't have its scimitar. 12 vs. AC probably misses. It looks resentful."
Eventually I had a player ask why I was so mad about them taking the scimitars. I had to explain I wasn't! I was just roleplaying the skeletons' frustration at finding themselves unarmed as they desperately tried to kill what they perceived as grave robbers.
I love to show the players the bad guys' feeling cocky or triumphant or proud, but I also love to show the players when the bad guys are feeling scared, disheartened, or frustrated.
What happened here is that the players were kicking the disarmed skeletons' asses and they assumed that I as the DM would feel frustrated. So when I acted for the skeletons, some of them thought it was me.
I should make a point to make sure my players know it doesn't bother me if the monsters are losing. On the other side of that coin, though, it doesn't bother me if the monsters TPK unless it's because of a bad rules call or house rule I made.
The best way I can communicate this to my players is the Beholder Story.
So in a certain module I ran, there is a boss fight against a Beholder. This guy is a deadly threat that should at least threaten to kill a PC or two. It's also an epic fight where lots of off turn attacks fly both from and toward the Beholder as it shoots eye rays on every PC's turn that starts even remotely close to it.
They evicted it from reality temporarily, then stun-locked it for the entire fight. It failed four saves in a row. I was tempted to fudge one of those, but I didn't. They kept it locked down and hacked it to pieces. It was a much easier fight, but you know what?
They were so thrilled! Every round it failed the save they were out of their chairs with glee! That's what I want for my players. That's why my skeletons were acting SO FRUSTRATED every time they missed!
I just have to make sure the players know it's not personal. It's me trying to show how awesome they are.