How much of a payload do you need to kill people? You can easily put a few hundred pounds of explosives in the aircraft along with a one way pilot. If a Kamakazi in a single seat fighter could sink a ship, a light plane pilot with similiar intentions could kill many given the right target. Is a lone bomber with a backpack bomb not effective because of a lack of payload? Tell that to the grieving in London. I am heading to an open air concert later this month, a 152 flown into the crowd with or even without explosives would be devastating.
Easy detection? Early detection didn't stop a C172 from violating Soviet airspace and landing in Red Square. As Saturn points out, a Cessna impacted the White House. Does a terrorist attack need to kill a load of people? No, it need only impart a sense of unease on the population along with the media storm that follows.