There is not a monster,but a boy who refuses to accept his mom leaving.
He knew the truth but he chosed to be treated in order to escape from the life withou his mom,which was scaring him and making him want to destroy everything ,and even himself.
The boy refused to admit the truth and relived his feelings by hurting himself and destroying things.He was afaid of the future without his mom.He was sad and confused.He thought if he refused the fate ,God would not let it happen.but death came to his mom at last.He understood his mother's heart and tried to open his heart to accept her leaving.
Lukily,a monster who was in his mother's stroies for him turns up and saves him from escaping behaviors.
A boy has to face many things as he grows up to be a man,such as people's leaving and so on.It is very hard and painfull for him to go through the sadess.