At first I thought the dragon was going to be shown eating Scales' corpse. I was like, "Ew... that's kinda sick."
But then I was like "Oh Scales is that small dragon right there"
Then "Scales shut up or you'll die"
Then "Oh shit surprise twist"
But the guy at the beginning said the halberd thing was cursed, and shed innocent blood. If Sintel had used a proper sword and not a cursed halberd, she would have realized what was going on. That's the moral of the story. Don't use halberds, they don't have any mechanism against friendly fire.
Also, I would have turned back after almost getting my ass handed to me by a guy with a halberd. A dragon is much more difficult to fight, even if you have the element of surprise. You still have a terrain disadvantage, and size, and firepower, and armor.