The post and caption are a reference to The Stranger, by Albert Camus, in which the protagonist Meursault shoots and kills someone, and in court says he did so because of the sun.

Ontariotaku said in comment #2586651:

The post and caption are a reference to The Stranger, by Albert Camus, in which the protagonist Meursault shoots and kills someone, and in court says he did so because of the sun.

Also, Meursault gets beheaded at the end of the book which his other identities reference.