What's the feeling about the hundreds of thousands of innocent civilians
that were killed as "collateral damage" by the US military¿
Did it help the people in those countries in any way that the US military is there¿ No. Those countries are still a mess and probably even more so than before the US invasion. Iraq, even though Hussein was a tyrant, was a secular state and Christians could live there quite savely, now Christians have to fear for their lives.
There is nothing to be sad about Saddam Hussein or Osama bin Laden being killed. Hussein was a tyrant and bin Laden certainly a terrorist and complete maniac. Both were trained and supported by the US to serve as political puppets though (Hussein in Iraq against Iran, bin Laden in Afghanistan against the Soviet Union) before they fell into disgrace.
Would it only have been about those two (and their close surroundings), the US could just have sent a kill squat and kill those guys (finally they did with bin Laden). It was not about them being "bad guys", it was about them not playing the puppets anymore.
It's a well known fact that those weapons of mass destruction, George W. Bush claimed to be in Iraq, which were the final "reason" to invade that country, were completely made up by the Bush government. One of the real main reasons to invade Iraq was, because Hussein threatened to cut down oil delivery to the Western states and mainly the US. That's like going into an apple store and killing the shop owner, because he doesn't want to sell you an iPhone.
Japan in WW II is a different issue. Japan was an ally with Nazi Germany and actually doing war in other countries, so fighting Japan was justified, but still it was NOT justified to drop those nuclear bombs on Hiroshima and Nagasaki, because that was willingly and PURPOSELY killing hundred thousands of civilians, and that wasn't even "collateral damage".