Betrayal
The theme of betrayal appears at several points throughout the text. First, Belbo and Casaubon experience a moment of reconciliation when Belbo admits that he often felt intimidated or manipulated by Casaubon in keeping with the liberal student movement, although he previously had kept that feeling to himself. Casaubon feels somewhat betrayed by the revelation, but he respects Belbo enought to move past the secret. In turn, Casaubon is accused of having betrayed Amparo. When she falls into a trance, she blames Casaubon for her bad experience because he was supposed to keep an eye on her. Traumatized by the experience, she directs her pain at him, lashing out in pathological ways. Ultimately this accusation drives them apart. Of course, the final betrayal is Allier's, who infiltrates the hospitality of Casaubon and his friends by feigning a mere casual interest in the Templar mystery when in reality he was pursuing the same quest. He gives himself away in a dramatic twist at Foucault's pendulum, eventually betraying Belbo specifically to the point of murder.
Madness
All of the characters of the book demonstrate a certain propensity toward madness of some kind. Casaubon and his friends understand their own vulnerabilities well enough to recognize the dangers of indulging obsession. When they become involved with the Templar conspiracy, they open themselves up to a whole array of new temptations. Most importantly, Allier and his companions teach Casaubon and the others a valuable lesson about themselves. They are criminals, alchemists, cultists, and extremists -- all suffering from some variety of insanity be it psychopathy (Allier), obsession, schizophrenia, lunacy, deviancy, etc. By the time Casaubon, Diotallevi, and Belbo recognize the dangerous nature of the men around them, they are forced to confront their own propensities to madness. After all they have pursued this conspiracy to the point of physical harm, at the expense of safety, relationships, security, financial stability, and mental health.
Violence in Relation to Boredom
As a college student of this time, Casaubon is no stranger to violence. The students were revolting in the streets, blackmailing people, and making violent demonstrations against the government in the 70s. Their actions were motivated by lack of purpose. Unable to find work but without any real resources to be successful in college, they were bored, just bored enough to turn violent. Similarly, Casaubon encounters this same strain of violence behavior in Allier who appears to be normal at first. Allier tricks the three friends into believing he is a casual, academic observer of the occult, but in reality he is a fanatic determined to uncover the secrets of the Templars. He constructs an entire identity around manipulating people to get closer to this goal. As he grows more complacent, he matches this inaction with violent anger, eventually threatening Casaubon and Belbo. By the end of the book, he is responsible for Belbo's death, along with a slew of other crimes, each more devious than the next, reflecting his own complete devotion to this goal. His actions are the product of extreme intelligence devoid of motivation. If he had something more productive to occupy his intellect, he may not have become a villain.
