A few months before Osama bin Laden’s death, Web sites linked to al-Qaeda ran excited commentary about a proposed new killing machine dubbed the “human lawn mower.” The idea was to attach rotating blades to the front of a pickup truck and drive the contraption into crowds.

While some jihadists admired the idea, one graying veteran of the terrorist movement took a stand against it. That was bin Laden himself, by then living out his twilight years in a Pakistani villa with ample time to think about his legacy. The man who famously ordered jetliners flown into skyscrapers drew the line at cutting down humans like weeds.

“He was upset about it,” said a former U.S. intelligence official who viewed bin Laden’s writings on the subject, part of a trove of documents seized from the terrorists’ compound in Pakistan a year ago this week. “He felt it conflicted with his vision for what he wanted al-Qaeda to be.”…

Bin Laden, in his missives, displayed an increasingly legalistic interpretation of whether a terrorist act is permissible under sharia. When Pakistani American Faisal Shahzad tried to detonate a car bomb in New York’s Time Square in May 2010, his attempt, widely hailed by jihadists, drew a surprising rebuke from bin Laden, who took a rare break from his self-imposed seclusion in central Pakistan to denounce Shahzad.

It wasn’t the prospect of civilian deaths that upset bin Laden, but rather the fact that Shahzad had planned the act after swearing a loyalty oath to the United States as a newly naturalized citizen.

“You know it is not permissible to tell such a lie to the enemy,” bin Laden wrote, according to a copy of his missive obtained by Jones, a senior political scientist at Rand Corp. (Bergen also refers to this incident in his book.)