We don't yet know the motivation(s) of the brothers involved in the Boston bombing. There are reports that Tamerlan Tsarnaev, the older brother, was an isolated and embittered young man. We don't know when or why that presumably happened. We don't really know if this is true, as there have been contradictory statements about him.
He reportedly returned to Chechnya and Dagestan last year, after living in the U.S. for many years. Both regions have militant separatist movements, and a history of having been at war with Russia. There are now reports that Tamerlan Tsarnaev returned to his homeland because of disenchantment with life in America. There is speculation that while there, he may have become "radicalized."
In other words, he may have become a zealot, motivated to engage in horrific acts because of some distorted ideology instilled in him while in his homeland. Assuming he was driven by some powerful religious or political belief, at what point do we separate the notion of zealotry from that of insanity?
Let's define the terms.
Zealotry is characterized by undue or excessive devotion or fanaticism. It can certainly lead to extreme, violent, and even murderous behavior.
Insanity (psychosis) occurs when someone loses touch with reality, and can no longer distinguish fact (reality) from fantasy. One of the primary hallmarks of psychosis is the state of being delusional.
A delusion is a false, fixed belief to which a person adheres, despite evidence to the contrary. The belief is not shared by others in one's social or societal group. There are many kinds of delusions. The most commonly seen are delusions of persecution. Delusional or psychotic beliefs can lead to violent, even murderous behavior.
When it comes to terrorism in which the Tsarnaev brothers were allegedly involved, was it driven by zealotry? Did they hold extreme and distorted beliefs instilled by exposure to virulently anti-Western or anti-American teachings?
Or, had one or both brothers become delusional, unleashing murderous psychosis on Boston last week?
Speaking as a psychiatrist, I can only say this question cannot be answered now. The older brother is dead and the younger one lies intubated in a hospital. He's now able to communicate by writing. Perhaps in time, we will better understand the reasons for what happened on Patriot's Day, 2013. Whether it was zealotry, insanity (or both) the brothers are completely responsible for their heinous acts.