Since he would been killed pretty quickly with the item that smashed him in his cage resting on his head and since you didn't find him like that, obviously an object didn't do it.
1. Someone smashed his head closing a lid,
2. Someone smashed his head on purpose and just left him in the cage.
So now my question for the OP is... Who did it?
My guess is Colonel Mustard in the library with the candlestick.
I really don't think enough information was given to use words like "obviously" or "I KNOW". It's a little soon to start accusing people, in my opinion.