As usual, I think BooMan has expressed what I feel extremely well.
Here's my problem. Based on what the government says it knew, I can understand that they did not want to leave this man free to continue his activities. It's our government's responsibility to keep us safe, and this man was quite dangerous. I also understand that capturing him was impractical. It's a highly unusual situation that our laws are not presently designed to address. I don't know if a similar situation will ever present itself again, but we need to craft our laws in a way that can account for this type of situation so that there is some legal review of some sort before a U.S. citizen can be assassinated by his own government.
Because, let's face it, without any legal process, the government could manufacture evidence against a U.S. citizen who is a harsh critic and who gives sermons that incite people against U.S. policies. We don't know what the line is where the government can disregard a citizen's rights, declare him an enemy of the state, and kill him. Even if we agree that al-Awlaki crossed it, we don't know at what point he crossed it. And it's not a simple question to answer.
As much as we all hate to admit it - Obama will not always be our President. While I can be happy that this man is no longer a threat to people and am aware that its important for our elected officials to take their responsibility for American security seriously, I also know that we are a nation of laws and not men. We can't rely on trust of those currently in power because gawd knows we've been very capable of electing some criminals in the past.
So this isn't necessarily a criticism of President Obama as much as its a recognition of the inadequacy of our laws. But then this is an area I always find troubling. For example, I'd take this even farther than BooMan has done. Is it just U.S. citizens who shouldn't be targeted for killing without any oversight? Can an administration otherwise kill people who are citizens of other countries simply because they determine them to be dangerous? That certainly was the case with Bin Laden and most of us didn't have a problem with that. But I can imagine situations where I would see it differently.
But perhaps I'm just not good at this kind of thing. I've always gotten uncomfortable with talk about things like "the rules of war." I recognize its necessary. But there's some kind of absurdity at work in crafting rules for the act of killing other human beings. So the idealist and realist in me really struggles with all of this and I don't have the answers. All I know is that there are important questions here that we shouldn't shy away from. And I, for one, am not willing to cede this ground to the Greenwalds of the world.