The GM is responsible for the thoughts and actions of every single being in the entire world besides the four to six player characters. Saying &quot;teehee I didn't kill you, grug the troll did :^)&quot; is incredibly silly. You're not playing a videogame or MMO, they can't just decide to turn their character around and walk away or fucking bubblehearth or Mario BLJ out of trouble, the fact they encountered the troll in the first place might not even be possible to avoid. If you decide they will encounter a troll in their travels from point A to point B there's nothing they can do about it, the universe does not actually exist outside of what you narrate to the players.