Not very many wars have no civilian casualties. And it's a shame the innocent get dragged into wars.
That being said, justice was done. You said it yourself, Osama was self incriminating. You can't get much more direct than that. Regardless of what you think of how trustworthy the U.S. govt is..
If you admitted to being the mastermind behind a 9/11 style attack, does it really matter how trustworthy the govt is? No. You just admitted to the crime. You are guilty.
What is more interesting is how outsiders try interpreting the U.S. Constitution. Because the fact is, there is more than just the U.S. Constitution that is called into play. Explain how the U.S. Constitution applies to anything that happens outside the U.S..
As for your quarter of a million Iraqi deaths...according to Wiki Leaks its quite a bit less than that. And of those is there any actual way to confirm who was or wasn't a terrorist. Especially since terrorists like to blend in with and drag civilians into the conflicts knowing it will create sympathy in their favor if civilians are killed. Blending in with civilians is good stealth, but should any of them get killed in conflict are also now sympathy pawns the terrorists use to gain more followers and those sympathetic to innocent civilian deaths.
Because now you have people not wanting (rightfully so) more civilian deaths, which also now can cripple how we attack the enemy especially if they hide in with civilians. It also can turn what could be a quick fight into a more drawn out one that results in more causalities. Nature of the beast. Sometimes you get unintended consequences from such actions.
http://www.ict.org.il/ResearchPublications/tabid/64/Articlsid/432/Default.aspx
Is One Man’s Terrorist Another Man’s Freedom Fighter?