What you fail to understand is that this would have decimated smaller countries. A country expended 18 people to not only kill thousands of people but to immobilize a nation.
If the retribution is not on a massive scale, I'm sure that you will agree, that it is a tactical victory for a country to expend 18 people in the future. A trial is not going to deter future actions. The tactical loss by your plan is very small, and will not prevent future attacks. I'm afraid that a large show of force on a population that cares nothing about civilians, is necessary.
And I love when people say that we have a high opinion of ourselves. We dole out aid and protection, and we are crapped on by the rest of the world and that we should be very calm when faced with the worst attack that we have ever faced.
If it had not been for the US having such big opinions of where it belongs in wold politics, I'm afraid that you would be speaking German and goose stepping down the streets.