How does an immigrant kid, who was basically raised in America turn out to hate us so much that he'd plant a bomb next to an 8 year old boy at a sporting event? I have an idea.
When I was a kid in school, I was taught that Christopher Columbus was a heroic man and that the Founder Fathers were, brave, principled people, who risked all in the name of liberty. I learned about the ingenuity of great people like Alexander Graham Bell, Thomas Edison, the Wright Brothers, and Henry Ford, who created the modern world. Yes, we also learned about America's tragic past with slavery. But we also learned that slavery was not unique to America and that many societies had people who suffered under slavery.
That's not what they teach kids today. Instead, kids today learn that A.) just about everyone important in American history, prior to Martin Luther King and the Kennedy's, were evil, white guys, who were hell bent on exploiting everyone else to enrich themselves, B.) America is still a racist country, C.) we have an unfair share of the world's wealth and use an unfair share of the world's energy, D.) our success was based on making others poor, and E.) we still refuse to atone for past and on-going sins.
The sad reality is that the entire notion of America as a "melting pot" been undermined by a leftist political, education, and media establishment that ferments racial and class divisions to further their own perverted aims.
Combine this constant derision of America by the official government establishment with an outside influence (like a radicalized older brother or Imam) and that's how you get a 19 year old American kid blowing up children with homemade bombs.
The reason the "public" school system was created in the first place (in Boston!) to was "socialize" kids into society (teach them how to be good, law abiding, Americans).
Today, the public schools are teaching kids how to be anti-American, or at the very least teaching them to be ashamed of America.
There isn't another country in the world where the establishment vilifies their own country to the degree that we do here. Germany does a great job teaching their kids to be ashamed of the holocaust. But, they also teach their kids to be proud of their country prior to and after that horrible period in their history. In spite of this huge stain on their history, the Germans know that the people of their country have created great music, art, literature, architecture, and economic success. Radical leftists in America refuse to admit that there is anything good, let alone great about America. They are the ones teaching our kids.
Recent American immigrates still have an idealized vision of America as the land of dreams. My wife certainly does. Once their kids get exposed to the leftist schools and other parts of the leftist culture, things can change quickly.
Given the circumstances, we should be thankful and a bit surprised that there aren't more unhinged young people running around blowing things up.