Does that consist of getting just all the gold stars in levels or you need to kill every monster in addition? Or something else as well?

The rules are usually created by the community. But 100% means full completion, therefore if there is a counter like "Monsters Killed" or "Gold Found" must be completed to its maximum potential (not including glitched or unreachable items like the level Ruins where you can only get 101/102 ammo because of a misplaced ammo box.).