The painting in question is the beheading of St John the baptist by Caravaggio.
Here we see two things in conflict(as always have and will remain) evil represented by darkness and the good represented by light.
In this ciaroscuro painting, at the back on the right hand side, we see two prisoners jailed, looking through the window bar with heads within a slightly fading spot of light and their background in complete darkness this could easily represent the position a human being find himself after an act of evil, putting oneself into isolation or finding one self into isolation as distant oneself from goodness. Caravaggio could easily be identified by this mentioned isolation as after a quarrel and seriously injery reaching the death of a police officer, he fled Milan in direction to Rome to find protection from authorities in mid 1952. A phrase from his biography says he arrived to Rome 'naked and extremely needy...without fixed address and without provision...short of money'.
His friend Longhi introduced him to the world of Roman street brawls, and most of the time he got into serious trouble which lead him to move from a place to another in Italy.
His attitude started to becoming bizarre sometimes sleeping fully armed and with his clothes, ripping up a painting at a slight word of criticism, mocking local painters and fleaing from a state to another when he got in serious trouble.
The two figures in the painting the executor and his victim in this case St John are lighten up from a dark background putting them in focus of the composition.
The executor can easily be identified with the force of evil and St John representing rightousness, but the light.
available at: http://www.caravaggio-foundation.org/[Accessed 22nd December 2012]
available at: http://upload.wikimedia.org/wikipedia/commons/d/df/Michelangelo-Caravaggio_021_Beheading-of-St-John-the-Baptist_cropped.jpg[Accessed 22nd December 2012]