The former US Navy SEAL who fatally shot Osama bin Laden has only one regret: the al Qaeda monster he killed with three shots to the head was buried at sea. “I would have hung him from a bridge in New York City,” and let the locals deal with him, Robert O’Neill told The Post. Here the retired Team Six member offers T he Post’s David Spector a riveting blow-by-blow account of Operation Neptune Spear, which unfolded 15 years ago today at bin Laden’s secret compound in Abbottabad, Pakistan . He also reveals the heroic operators’ 9/11 motivations. “We were going for the single mom who dropped her kids off at school on a Tuesday morning, then an hour later . . . jumped out of the World Trade Center, pressing down her skirt as her last act of human decency. She was never supposed to do that.” We first found out about the top-secret mission three weeks before it was executed.…