Bin Laden worried that the "human lawn mower" "conflicted with his vision for what he wanted al-Qaeda to be."

"The idea was to attach rotating blades to the front of a pickup truck and drive the contraption into crowds."

This is what WaPo forefronts in the article headlined "Bin Laden’s last stand: In final months, terrorist leader worried about his legacy," but if you keep reading to the very end, you'll see that the main image problem that troubled him was false oaths:

When Pakistani American Faisal Shahzad tried to detonate a car bomb in New York’s Times 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... Complaining of the “negative effects” to al-Qaeda’s image, bin Laden noted that jihadists already were under suspicion in parts of the world for “reneging on oaths, and perfidy.”