What is in question here is not whether the state can in some circumstances kill, and if this was one of those circumstances (that question comes later).
The preliminary question is whether the state is allowed to assassinate citizens without trial outside wartime, and refuse to reveal why. That's the position of the Obama administration - you have no right to question their assassinations, you must simply accept that the government knows who, when, and how to kill. That's the question Rand Paul is asking in this essay (and a few others have asked before him), that's why he says 'show us the legal case', and talks about the rights of all American citizens to a trial by jury rather than 'this action was wrong'.
If you accept that the state has the power to kill without justification or investigation (before or after the fact), a lot of the other rights you are supposed to have as a citizen become meaningless. The rule of law (especially restricting the state) is vital to protect citizens without the resources to fight the state. That's why this first question is so important, whatever you think of Anwar Awl Alaki. The rule of law restricts the activities of the state and makes it less powerful and effective, that is as it should be, so you shouldn't argue that this makes our government less able to kill people quickly - that's the point.
Moving on to the question you address of whether it was actually legal or justified - typically killings outside the judicial process and outside of wartime are viewed as assassinations and very difficult to justify; I'm not aware of evidence that Anwar Awl Alaki was planning a particular attack or involved in one, and the killing of his son a few weeks later along with some other teenage friends is based on even more tenuous evidence and justification (if any). We didn't mean to kill him is not a reasonable excuse for a government agency outside of war, and to kill someone like a hostage taker a government agency would have to submit to an investigation afterward. Killing without trial or investigation means your government can assassinate citizens at will.
The preliminary question is whether the state is allowed to assassinate citizens without trial outside wartime, and refuse to reveal why. That's the position of the Obama administration - you have no right to question their assassinations, you must simply accept that the government knows who, when, and how to kill. That's the question Rand Paul is asking in this essay (and a few others have asked before him), that's why he says 'show us the legal case', and talks about the rights of all American citizens to a trial by jury rather than 'this action was wrong'.
If you accept that the state has the power to kill without justification or investigation (before or after the fact), a lot of the other rights you are supposed to have as a citizen become meaningless. The rule of law (especially restricting the state) is vital to protect citizens without the resources to fight the state. That's why this first question is so important, whatever you think of Anwar Awl Alaki. The rule of law restricts the activities of the state and makes it less powerful and effective, that is as it should be, so you shouldn't argue that this makes our government less able to kill people quickly - that's the point.
Moving on to the question you address of whether it was actually legal or justified - typically killings outside the judicial process and outside of wartime are viewed as assassinations and very difficult to justify; I'm not aware of evidence that Anwar Awl Alaki was planning a particular attack or involved in one, and the killing of his son a few weeks later along with some other teenage friends is based on even more tenuous evidence and justification (if any). We didn't mean to kill him is not a reasonable excuse for a government agency outside of war, and to kill someone like a hostage taker a government agency would have to submit to an investigation afterward. Killing without trial or investigation means your government can assassinate citizens at will.