The monstrous, then the mundane.

The Boston marathon bombs went off about 2:49 p.m.

At 3:13 p.m., not even a half hour later, admitted bomber Dzhokhar Tsarnaev appeared on a security camera across the Charles River at a Cambridge Whole Foods market. We watched him on screen in the federal courthouse Monday.

For several seconds in the dairy aisle he held a half-gallon of milk, studying it. He put it back. He took another half gallon, studied that, walked to the checkout — all caught on camera — and handed the cashier a $20 bill for a $3.49 half gallon of High Lawn whole milk, lightly pasteurized, all natural, from the Massachusetts Berkshire hills.

Seconds later he ran back inside, apparently telling the cashier he needed an exchange. He picked out and studied yet a third half-gallon, then left again, this time getting into the passenger seat of a waiting car driven, presumably, by his brother Tamerlan.

It left some of us watching startled: who frets like this over buying milk? Who could even think of milk minutes after doing what he and his brother just did?

“Islam is a religion of peace.”

I’d heard these words — again — just days before this trial testimony at a Boston College lecture, “Islam for Catholics 101.”

As the Tsarnaev trial was beginning, Natana J. Delong-Bas, an assistant professor in the college’s theology and Islamic Civilization department, said she hoped to correct some media misinformation about Islam and dispel the idea that it breeds terrorism. She wanted to show its similarities to Christianity.

Among the many: Muslims believe the prophet Muhammad and Jesus will one day rule together. They believe in Jesus (and Noah, Abraham, Moses, and David) as prophets, though they do not consider Jesus the Son of God. They believe the angel Gabriel not only visited Muhammad but, like Christians, that he also visited Mary to tell her she’d be Jesus’ mother and Zechariah to tell him his wife would give birth to John the Baptist. The stories of Zechariah and Mary are both in the Quran, which speaks of Jesus’ virgin birth and of Mary herself more than the New Testament does.

Like Jesus, Delong-Bas said, Muhammad was taunted, mocked, and threatened with death for his beliefs. His followers, like the early Christians, were persecuted, tortured, and killed. Both religions believe in a judgment day, an afterlife, Adam and Eve, and one God as the source of all life.

Of course Muslims, unlike Christians, believe Mohammad was the final and mightiest prophet of God, and that the Quran is God’s final word.

“Violence is forbidden to the Muslim community” save for self-defense, Delong-Bas said, underscoring her theme: Islam is not what causes these terrorist atrocities.

Yet Muhammad himself not only led armies and conquered peoples, but also, for example, ordered the assassinations of some who satirized him and permitted the execution of prisoners, the selling of women and children into slavery, and the harsh treatment of those who would not convert to Islam.

Asked after the lecture if extremists look to such actions to justify their own, Delong-Bas noted the often-brutal violence of the Old Testament God. She also said what religious leaders, Muslim and non-Muslim, often do: that those who embrace radical Islam distort and do not really understand their faith. They are often troubled, alienated young people ripe to be seduced by charismatic ideologues offering some grand purpose to their lives.

This is not a very satisfying explanation to the still unanswered question hanging over the Boston bombing trial: how a well-liked, non-alienated, then 19-year-old pot dealer morphed into a cold-blooded killer, no matter how many terrorist lectures he downloaded on his laptop.

In fairness, Delong-Bas was not trying to explain the particular radicalization of Dzhokhar Tsarnaev. But as this trial progresses, it’s increasingly clear that avenging fellow Muslims killed by the American military was Tsarnaev’s motive.

Tsarnaev tweeted in the days before the attacks, “Strive to be a better muslim, be greedy with your time. Devote most of it to the Almighty for it is his satisfaction that you need.”

And this, “Listen to Anwar al Awlaki’s,” the American-born Al Qaeda radical, and “you will gain an unbelievable amount of knowledge.”

Tuesday in court prosecutors showed jurors the neatly penciled note a bleeding and wounded Tsarnaev printed inside the boat where police in Watertown, a town just outside of Boston, captured a young man who placed a bomb behind three children, killing one, nearly killing another. It reads in part:

“God has a plan for each person. Mine was to hide in his boat and shed some light on our actions I ask Allah to make me a shahied (a hero in heaven killed unjustly during “Jihad”) to allow me to return to him and be among all the righteous people in the highest levels of heaven. He who Allah guides no one can misguide …”

Since this trial began, reporters have noted that Tsarnaev sits in court appearing disinterested and unmoved, even by heartrending testimony from those who lost legs or saw friends — or an 8-year-old son — dying before their eyes. The admitted bomber shows no remorse, they report. Perhaps because he thinks he did no wrong.