Why did the suspected Boston bomber pivot from benign to brutal?

Dzhokhar Tsarnaev, the younger of the two brothers accused of perpetrating the Boston Marathon bombing, is the baffling mystery man in this crime.

His older brother, Tamerlan, who died in a shootout with police in the dark early hours Friday morning, better fits the stereotype of a disaffected, nascent terrorist.

He was nearing adulthood when he came to this country from Russia’s predominantly Muslim central Asian region. He talked of having no American friends. He had openly disdained the immorality of American society and adopted a zealous brand of Islam. He had left school and was in a troubled marriage.

On social media, he had connected to sites touting extremist Muslim ideology. He had traveled back to Chechnya and Dagestan, where he conceivably could have met with and been trained by terrorist groups. The Russians had asked American authorities to check him out, prompting the FBI to question Tamerlan and his family in 2011.

It’s not hard to concoct a scenario for Tamerlan that ends with a bombing. But Dzokhar?

Seven years younger than Tamerlan, Dzokhar came to the U.S. when he was 8 years old. Recently, he became a citizen. In between, he lived a relatively normal American life.

He was a successful student and competed on his high school wrestling team. He had many friends. Those friends say he was upbeat, always smiling. He was enrolled at the University of Massachusetts-Dartmouth. He lived in a dorm. He went to parties. He worked out at the gym.

To one teacher, he did express an interest in the troubled history of his distant family homeland, but he seems to have been happy living a well-adjusted American life.

How could Dzokhar have pivoted from that to the vicious act of which he is accused?

It is good that police were able to apprehend him alive. If and when he recovers from his serious wounds, Dzokhar may be able to enlighten us about his apparent turn to heartless violence. In the meantime, there will be plenty of speculation.

Conspiracy theorists will imagine that he is some sort of sleeper agent for Muslim terror groups; that his sunny demeanor concealed a sinister, coldblooded intent. It could be true, but I suspect the reality is much simpler.

Dzokhar is 19 years old, an age when many young men act on impulse and sudden passions. It is the age when boys trying to prove their manhood are easy to recruit. It may be to the military, it may be to religion, it may be to the brotherhood of a hard-drinking fraternity or it may be to a cause that promises them the chance to change the world.

Perhaps, most of all, it is an age when someone older with a firm vision of life can have an inordinate influence. It may be a professor, a drill sergeant, a religious figure -- or an older brother.

When we finally learn the truth about this alleged bomber, the shock may not be that he is so sinister or so consumed by a cause. The shock may be how easily this young man -- like many young men -- could be swayed from the light to the most brutal darkness.