Some random thoughts (and the spoiler):
Fear can bring out the worst of us. Everyone of us live a monster in ourselves, it's just if our monster will be awaken one day. But we are free to choose if we let the monster sleep or wake it up.
Does anyone notice that Johan likes to raise his hand like the "monster" who took his sister (though he thought it was him) away? Victim can become a perpetrator by imitation!
In order to deny the world, we either kill others or kill ourselves.
For me, the most heartbreaking moment is the last chapter (The Real Monster) which explains why Johan develops into a monster. Despair and loneliness of the abandoned.
The empty bed, Johan is dead? He was going to find his mother? He escaped again to do more evil deeds? It's up to your interpretation. Of course I personally want to believe that he walked out to find his mother, but we don't know. Maybe it's like the Life of Pi, you choose the ending. After all, it's the life that we make the choice.
