The death sentence for Boston Marathon bomber Dzhokhar Tsarnaev may provide some kind of justice for the families of the four people that he and his older brother Tamerlan murdered, as well as the 16 victims who lost limbs in the bombings. But the decision gets us no closer to the big question: Why do this? Why set off a bomb that kills Martin Richard, an angelic 8-year-old boy? Or Lingzi Lu, 23, a graduate student in mathematics from China?
According to Tsarnaev's own writing as the police closed in on his final hiding place, a boat dry-docked in a backyard in the Boston suburb of Watertown, it was about U.S. foreign policy in the Muslim world.
Losing blood rapidly as he lay wounded from multiple police bullets, Dzhokhar wrote on the inside of the boat with a pen: "The U.S. government is killing our innocent civilians, but most of you already know that. As a M[uslim] I can't stand to see such evil go unpunished. We Muslims are one body, you hurt one you hurt us all...Know you are fighting men who look into the barrel of your gun and see heaven. Now how can you compete with that? We are promised victory and we will surely get it."
This was an efficient rendering of Osama bin Laden's essential message that Islam is under attack by the United States and true Muslim believers who "love death" must take revenge.
So part of the "why" for Tsarnaev was the ideology of "Binladenism" that has been widely disseminated around the globe since the 9/11 attacks.
What's puzzling is that Tsarnaev was hardly an observant Muslim. As a sophomore at UMass-Dartmouth he was an easygoing, skateboarder dude, and had a reputation for partying. Twitter and Facebook accounts document Dzhokhar hanging out at a wide range of parties.
He was also a prolific tweeter. His tweets generally had almost nothing to say about the practice of Islam or American foreign policy in the Muslim world, which are the preoccupations of many militant Islamists.
Instead, they were the typical musings of an indifferent 19-year-old American college student: Homework that was late, sleeping in, sex, girls, marijuana and alcohol.
A not untypical tweet from Dzhokhar six months before the Boston bombing said: "This night deserves Hennessy a bad bitch and an o [ounce] of weed the holy trinity."
Hennessy, "bitches" and weed are not, of course, the preoccupations of the typical Islamist zealot. In fact, it's hard to find any evidence that Dzhokhar believed in much of anything.
His involvement in the world of anti-American jihadism seems to have been precipitated largely by his older brother Tamerlan's deepening Islamist beliefs, although the argument he was overly influenced by his brother clearly did not stop the jury finding him guilty of the murders he was charged with and sentencing him to death.
Still, Tamerlan was the star of the tight-knit Tsarnaev family. He was the big and strong son. His mother adored him and his father, a onetime boxer, encouraged his interest in the sport. But for someone who dreamed that one day he would become a world-class boxer or musician, who drove around town in a white Mercedes (a perk from his father Aznor's car repair business) and dressed in the style of a small-time Russian pimp, favoring skin-tight leather pants, pointy shoes and billowing white shirts open to the navel, Tamerlan's actual life was somewhat less glamorous -- he dropped out of community college and then worked as a pizza delivery guy.
And, as he began to explore the Islam native to his family's homeland in the former Soviet Union, Tamerlan became increasingly disenchanted with life in his adopted country.
In a photo essay about Tamerlan's boxing career that was published in 2009 on a site that helped photographers promote their work, Tamerlan volunteered that he felt alienated from his adopted country, saying, "I don't have a single American friend. I don't understand them." Tamerlan also said he was "very religious" and that he had quit drinking because "God said no alcohol." In the photo essay, entitled "Will Box for Passport," Tamerlan explained that he wanted to get on the U.S. Olympic boxing team and also become an American citizen.
In 2010, Tamerlan won the Rocky Marciano Trophy, which made him the Golden Gloves heavyweight champion of New England. He was now one of the top amateur boxers in the States, but Tamerlan was blocked from advancing to the national championship of the Golden Gloves because of a recent rule change that prohibited non-U.S. citizens from competing. And Tamerlan's penchant for violence outside of the boxing ring was going to make it hard for him to obtain an American passport.
At the same time that his boxing career was blossoming, Tamerlan for the first time began attracting the attention of law enforcement. On July 28, 2009, Tamerlan hit his live-in girlfriend. "Yes I slapped her," Tamerlan told the police when they interviewed him about the incident. The case wasn't prosecuted, but it likely held up his later application for U.S. citizenship,
Around the same time, the family patriarch, Aznor, got in an altercation at a Boston restaurant and was struck in the head with a steel pole, an injury he never quite recovered from. Then Aznor's car repair business took a dive.
The family now subsisted on food stamps and welfare payments. The parents divorced and Aznor went back to Dagestan, while his wife, who had once dressed in high heels and tight skirts, adopted a veil and found a new identity in fundamentalist Islam. In 2012, she was accused of shoplifting $1,600 worth of dresses from Lord & Taylor. She fled to Russia and records suggest she remains wanted on a felony charge.
Bella and Ailina, the couple's daughters, moved out of the family home in Cambridge, settling in New Jersey where Bella was subsequently arrested on a charge of distributing marijuana and she entered a pretrial intervention program.
The Tsarnaev family was fracturing.
In June 2010 Tamerlan married Katherine Russell, his on-and-off girlfriend. They had met by chance in a downtown nightclub. Russell converted to Islam and took a new Muslim name, Karima, and started wearing the body-enveloping burqa. The couple soon had a baby girl.
Tamerlan and Russell lived on welfare payments for a while, but those ran out. Russell started working as many as 80 hours weeks as a home health aide, while Tamerlan was unemployed.
Once a possible contender for the Olympics, Tamerlan was now jobless and dependent on his wife for any income. As he drew deeper into the world of Islam, Tamerlan quit boxing entirely and stopped wearing flashy clothes, occasionally wearing a bathrobe over his habitual uniform of sweatpants and T-shirts.
A schlub going nowhere, Tamerlan was now a very long way from his conception of himself as a larger-than life hero.
At a Boston mosque Tamerlan stuck up an unlikely friendship with Donald Larking, an elderly invalid who had recently converted from Catholicism to Islam. Larking and Tamerlan were fellow conspiracy theorists. Tamerlan believed that 9/11 was an inside job and that the government had pulled it off.
Tamerlan's mother, Zubeidat, reportedly also believed that 9/11 was engineered by the U.S. government to create mass hatred for Muslims. Her views of the "real" causes of 9/11 were shared by both of her sons.
In addition to his fringe beliefs about 9/11, Tamerlan also started recommending to acquaintances "The Protocols of the Elders of Zion," which purports to be the secret plan by the Jews to take over the world. It was long ago debunked as a century-old Russian hoax.
The YouTube account maintained by Tamerlan took on an increasingly militant tone. He posted a well-produced video about the prophesy that an Islamic army carrying black banners would appear from Khorasan -- an ancient name for a region that encompasses Afghanistan -- to bring true Islam to the Muslim world. The prophecy is a key part of al Qaeda's self-image as the black-banner-carrying army emerging from Afghanistan that is destined to bring back the caliphate. The video that Tamerlan posted to YouTube showed al Qaeda fighters training in Afghanistan.
Like many other jihadi militants in the West, the Tsarnaevs downloaded Inspire, the webzine put out by al Qaeda in the Arabian Peninsula.
The first edition of Inspire, which appeared in the summer of 2010, featured an article about making bombs from pressure cookers and gunpowder from fireworks, a bomb-making recipe that appears to have influenced the Tsarnaevs.
Dzhokhar Tsarnaev couldn't resist telegraphing his forthcoming role in the Boston bombings. For the first time in his young life he was going to be someone other than the slacker barely-getting-through college student and small-time drug dealer that he really was. In the weeks before the attacks, Dzhokhar tweeted to his followers: "If you have the knowledge and the inspiration all that's left is to take action."
The action Dzhokhar took at the Boston Marathon seems to have emerged from the lethal combination of the ideology of Binladenism, which has taken on a new life with the proliferation of English-language jihadi propaganda such as Inspire; his brother's disappointments in life; his deepening Islamist beliefs; and the collapse of the Tsarnaev's family life.
But this still doesn't really answer question "Why?" After all, plenty of people object to American foreign policy in the Muslim world and many people have disappointments in life, and many families fall apart, but few turn to violence for these reasons.
Two books have been written about the Tsarnaevs; hundreds of news articles have been written about them, including an excellent series by the Boston Globe; the trial of Dzhokhar Tsarnaev produced many thousands of pages of testimony, yet we are still no closer to having a totally convincing explanation of why the Tsarnaev brothers from Cambridge murdered their fellow citizens.
Perhaps this says something about the fundamental nature of evil acts; you can point to why a convicted mass murderer might have disappointments in life or be swayed by bin Laden's ideology, but the Tsarnaevs' acts are ultimately inexplicable.
Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad.