"Dastardly" was given as a synonym for cowardly and the definition for dastardly was:
1 : COWARDLY
2 : characterized by underhandedness or treachery <a dastardly attack> <a dastardly villain>

Obviously, 2 applies to the WTC tragedy.

But I agree with fiberbabe - it's hard to get your head around the concept. Sounds right until you try to puzzle out the actual meaning, then it's hard to put it precisely into words.