> exit waste storage area
You duck out of the passage and fire your light cannon straight at the grue king's throne. Your cannon illuminates the entire room just faintly enough for you to see that the grue king is not actually on his throne, but charging at you from your left. Oh, but at least you spotted the light switch while the room was illuminated. You reach over and hit the lights. The Grue King quickly explodes right before he would otherwise have eaten you.
Well, that was easy. You almost wonder why the Grue King had a light switch in his throne room anyways.... The room illuminated, you can now see everything.
The room to your left is labelled "prison" and the room to your right is labelled "Waste Disposal". Except you probably figured that last one out. Also there's what appears to be a miniature axe behind the throne, as well as a button on the throne. Also, for some reason, you can't go right.
> go to throne
You can see the button quite a bit better from here. It says "disable a trap".