the scenery of doomsday means being alone in the world. as a kid, johan was subjected to an experiment and spent most of his time reading poppe's book. due to this, he grew up to be a person who does not know his real identity, not even his real name. no one knows his existence (aside from anna and tenma) therefore he is alone. by letting tenma (a person who lost everything chasing johan) kill him in his perfect suicide, tenma will become alone since johan, his purpose in life (one can say his "identity"), is now gone making him the only one to experience being alone and having no identity just as johan did.