I love to study sociology because it helps me to understand the social problems of our society. In the recent bombing in Boston that killed several people and injured hundreds, many people ask, what was the motive? When I reflect upon it, I think of the teachings and writings of the classical French sociologist, Emile Durkheim, of the nineteenth century. He questioned such crimes that can occur in modern societies when immigrants are involved. The norms of a different society can bring conflict into the lives of the immigrants who have a difficult time in assimilating.
You can look at Wikipedia or pick up a book on Social Theory concerning the classical sociologist, Emile Durkheim, and see what might be explained as being the motive for the two immigrants in Boston, who allegedly bombed the people at the Boston Marathon. He calls the problem, "anomie, an imbalance of societal norms."
"According to Durkheim, anomie reflects a sense of normlessness, the lack of any societal norms that spurs the tendency to act in a deviant way. In general terms, Durkheim’s theory of anomie proposes that because of industrialization and the need for cheap labor in this newly modern society, the influx of immigrants inherently brought with them their own sets of norms and values. Thus came a temporary imbalance of norms, anomie, which enhances individual’s propensity to commit crime in search for a stable environment. In turn, Durkheim puts forth not just a theory for the social origins of crime, but also he theorizes about the social origins of law and punishment." (Wikipedia)
This is not to excuse the crimes committed. But simply explains what is. Although, Durkheim looked at societies over a hundred years ago, many sociologists use his theories today. Perhaps the two brothers who immigrated from Russia or Chechnya were disturbed by the newer values and norms of their new home and had a difficult time adjusting and reacted eventually in a gross criminal way.