Death
Despite the minor conveniences and comforts the Gladers have managed to establish, the Glade and the surrounding Maze carry a specter of death. Thomas feels this almost immediately upon arriving in the Glade. He only wants to leave and escape. Overcoming this constant sense of fear and trepidation becomes a key point to maintaining his sanity and mental well-being.
Thomas's discovery of the graveyard, and the morbid grave site of the young boy who tried to escape via the Box tunnel, only serve to further cast the environment as one in which life is constantly vulnerable. Awaiting the Gladers outside the protective walls is a world run by monsters where they would likely die violent deaths. The fear of painful and unpleasant deaths, rather than the fear of simply being dead, serves to control the children and limit the chances they are willing to take.