Justice, such that we do not destroy innocent lives.
Vengence, such that future terroristic organizations may think, "bomb the USA? <involuntarily pissing of pants>" Yes.
Unfortunately, we may have to kill innocent civilians as we right this wrong. That is unfortunate, but as Steve Stirling said, don't stand next to a person who slings shit at a person armed with a gun. He's going to take out the slinger, but you unfortunately may well be collateral damage.
We need to take out the people who did this slinging of the shit at the USA, and the WTC is a heap of shit slung at us, but I am not in the least concerned about possible damage about those who are standing close to the slinger. Give them 24 hours to give us the people responsible, then (if nonconpliance) utterly destroy them.
I note that as yet there has been no official determination as to who organized it. If it was bin Laden, Afghanistan will be hard. If it was Hussein, well, it was easy last time, it'll be harder this time, but if it can be shown either or both funded or participated, they should be wiped off the face of the earth. Make Iraq and Afghaniistan a sheet of radioactive glass if necessary.
Sure, inhumane. Sorry. Bashing planes into the former world trade center was, I would think, rather inhumane. If you are a government who sponsored a terrorist organization that did this, you'd damn well better cough up your information now.