As I said in my post, killing was necessary, but like Doc Millahh said, it was probably not an end unto itself.
Honestly, if body count was their primary concern, they really bungled their choice of targets. They had no way of expecting that the towers would actually collapse. That was just good fortune [on their part]. They couldn't have had much expectation of actually damaging the Pentagon very much, at all. The Capital could have been ugly [from the body count POV, at least], and in that instance, it would have been select casualties that they were after. Killing a big chunk of Congress would have been much more appealing to them (and us, for that matter) than a bunch of secretaries and bankers.