Of course! There is not only his words, there are witnesses etc... even from where they killed him they had documents of plans for more attacks. Look at Saddam, he was tried and hanged.Would he? If everything was done according to the law I don't suppose they'd find enough evidence - I'm not sure but I think the only thing they had was his own words? Also he'd probably have a lot of "harmful" stories to tell - I believe it's the main reason they wanted him dead.
My point is, is that it shouldnt be the US's call. They seem to decide what this world needs without actually asking them.
You might have a point on the 'harmful' stories part though