I made a dungeon, a downed transport ship that crashed on a planet. It's full of valuable treasures belonging to a Hutt. Ship crashed because of unknown reasons. Players are being sent to loot hit. The group is around level four. If they use lore or criminal contacts they find out that the treasures are the Hutt's private zoo. If they dig deep enough they find out that he had recently added a pet rancor to his collection.

They go into the ship killing creatures and having a great time, they started at one end and were working they're way through the ship front to back. Pretty linear dungeon but good fun. They get to the back of the ship. There is the rancor. I tell the players "This thing is huge and is the single most terrifying thing you've ever seen in your life. It's head turns towards you, and as it begins to move you note it is slowed down by an injured leg. Even though it is injured everything inside you is telling you to run for your life."

Now as a gm I thought the players would kite this thing back through the entire dungeon, get back to their ship, then finally kill it with the ship guns. It was supposed to be epic as it rips its way through wall after wall of the ship chasing them while they shot it over and over again. My dumb ass players charged the rancor. The entire party committed stupicide. Death by stupid.

I'm seriously wondering what I could have done differently if anything to get them to simply run away rather than complain and say I'm a bad gm.....