As the title of the anime Monster (along with the Biblical quote that appears in the first few episodes) indicates, this anime took influence from Judaeo-Christian mythos. The show manifests these influences via the use of subtle symbolism.
The main bits I caught had to do with the Mark of the Beast, which, Biblically, are marks that one has on his forehead or on his right hand and which symbolize his tendency to think or act out blasphemy respectively. They also have a relation with the Beast of Prophecy, basically a stand-in for the Antichrist.
In the anime, Johann Liebert is constantly shown pointing to his forehead whenever he wants someone to shoot him. Though you can take this at face value and not look for a deeper meaning, it is also impossible for me to overlook how he is basically the Antichrist of Monster's world and how his pointing at his forehead symbolizes his corrupt mind and, more importantly, how he is the mind behind most of the corruption and evil within the series.
And if he is the mind, then Roberto has to be the hand, the one who acts out Johann's schemes. He even has a mark on his hand to prove this point, a mark that comes in the form of a wound that he receives from Doctor Tenma when the latter shoots him in the library. And indeed, in the show, whenever you see Johann, he is mostly accompanied by Roberto, who does most of the work for him.
Think about it. It's Roberto who burns down the library, who kills most of the inhabitants of the town near the end, who shoots the man responsible for the child experimentation, who attacks the psychologist looking into Johann's acts, and it he who gets rid of one of the two corrupt police officers who helped Johann.
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