A flying mount and a Legendary dagger – this was already some extremely valuable loot.
However, when Water Fairy also looted a cloak, Lu Li suspected that Remnant Dream's spirit had been passing through her. After all, these things that were dropping were just too amazing. Anymore, and it would impact the fairness of the game.
The game developers probably didn't think that there really would be someone who would kill the Princess of the Black Dragonflight, Onyxia.
According to the normal game storyline, Onyxia would be beheaded by Varian, simply to emphasize his mighty power.
Varian didn't disappoint the game designers; he really did behead Onyxia in one go. However, when the blade of his sword cut off Onyxia's head, she had in fact already died.
She was brutally killed by those two little Thieves.