(You're the monster) there's a lot of reports of people sleeping and being killed by a unindentified entity (you) and Mark inspired in Mandela catalogue's mark lol, is one of the victims, he woke up, and he noticed a monster. Perfectly matching the descriptions he heard in the news... So he locks the door
Mark: Oh god-thats the thing! He quickly pulls and hugs his gun, that he always had on his very bedside cabinet, since he was always paranoid