Good discussion! I recently started to run the AP (before reading this thread) with an experienced set of players and I have to admit made this mistake. I let sheriff Hemlock set the bounty on goblin ears and the head of chief Gutwad. As soon as the players discovered the Goblin village, they started to kill the Goblins. I assume they are still stuck in this old stereotype thinking that Goblins are evil and can be killed without any remorse.
At least, two Goblins managed to escape and the party captured one alive to learn about what happened before. I hope they keep to their promise letting him go unharmed if he tells them everything. We will see in the next session.