" If you can take whatever action is deemed appropriate based on your beliefs regardless of secular constraints, then why can't others -- as long as they are genuine in their belief?"
In short - 'cause we're right and they're wrong. :o)
Again, the source of Western morality is Torah. We, in the Western world, take for granted concepts like "life is sacred above all else"
In Islam, as well as many Asian traditions, honor is more sacred than life.
Kamikaze pilots in World War II, suicide bombers in Israel, Muslim fathers killing their disobedient daughters in the U.S., these people aren't crazy. They just have a different value system.
If we take the secular view, then we truly can't say that any value system is inherently superior to any other.
I don't subscribe to that view. I say that we (of Judeo-Christian civilization) are right and everyone else (to the extent that their values contradict ours) is wrong.
I don't have the exact quote in front of me, but I heard Mr. Jillette say something in an interview, something about how he respects those with religious conviction much more than those in liberal churches (synagogues). He can say to them "you're dead wrong" and they can say to him "you're dead wrong", but at least they can have a dialogue and present arguments for each side.
Without an anchor, without an objective value system by which to live, we have no justification to tell anyone else that any given action is "right" or "wrong".
So, that's why it's wrong for people to hijack airplanes and fly them into buildings, even if they think they are serving some higher power by doing it.