It's possible we killed more Iraqis removing Saddam than he killed while in power. Are those deaths acceptable?
No we did not, the lie that a million people died has been debunked so many times it is a laughing stock. Iraq has the figures available for how many people died during and after the invasion and the on going war, by the way we are not responsible for the dead due to the efforts of terrorists to over throw the Country.
Assuming everything you said is true, how many lives ended by the US would have been too many to be considered justifiable?
Conservative estimates put CIVILIAN VIOLENT deaths at 250,000.
It worries me no end that people are still defending our intervention there as positive and justifiable. There is no calculation that maybe our best interests are not worth the lives of any number of others and to hell with the long term consequences. It's like we're stuck in a B western.
I agree, there are various other was that we could have "disposed" of Saddam, other that war. Supporting his adversaries, black-ops assassination, crippling sanctions (though this would not work without global support), just to name a few. Now, I am not suggesting any of these options would be better, or even condoning any of them. I am simply giving examples of how we could have "disposed" of him, short of war.
I am not even completely convinced that we needed to get rid of Saddam, he was a powerful balance to the Iranian regime. Toppling his regime does seem to have had the effect of de-stabilizing the region, though there are other factors.
To answer the OP question, there are times when passivism is not virtuous. Take the example of the woman being murdered, doing nothing would not be virtuous, it would make one complicit in the act. You could have stopped it, but didn't, why? Because your passivism is more valuable that her life? Open aggression by a nation towards ones' own nation would be another potential example, it would depend greatly on the circumstances. In the case of Japan bombing Pearl Harbor, passivism would likely have lead to further aggression and lost lives, and only served to delay the inevitable.