It wasn't just the USG; the United Nations listed al-Awlaki as a terrorist in 2010, one year before he was killed: http://www.un.org/sc/committees/1267/AQList.htm. The UN isn't exactly known for being a hawk...
Thomas has already covered this quite well, but I'll briefly summarize again. al-Awlaki was actively at war against the USG. Killing him is roughly equivalent to killing US citizens who fight for other foreign governments. The drone strike was not carrying out a legal sentence, it was simply the use of force in a global conflict.
The UN is exactly known for being a court of law whose jurisdiction compels the USA to mete out sentencing with no right of appeal either.
If he was, why was it too hard to run the case? Why all this apologist stuff about an accusation being good enough for a death sentence - also carried out on his son while he sat in a cafe a week later. This is Stalinist reasoning, what is it doing in the USA?
Trials in absentia are unconstitutional in the US, and al-Awlaki was embedded with AQAP, a paramilitary force far outside the reach of the Yemeni government, with which it is at war.
The logic of the strike was that waiting for that situation to resolve itself would have the effect of giving al-Awlaki many more opportunities to coordinate attacks, that the pattern of al-Awlaki's involvement in attacks was one of escalating intensity, and that the attacks plots themselves were becoming more ambitious. At some point, the logic goes, he'll figure out how to coordinate a strike that actually succeeds in creating mass casualties.
There are a lot of very valid reasons to oppose drone strikes. For one thing, you might believe Obama to be a fundamentally reasonable person, and then remember how awful Bush II's national security staff was, and think about what crazy things the next President will do with the capabilities. You might, like me, also categorically reject the death penalty. And you might also be concerned with the idea of a "declaration of war" on a terrorist brand name which any idiot in the world can revive and slap on a black flag any time they want, thus ensuring that the war is never brought to an end. I know that's what I think.
What I don't think you can say is that in the al-Awlaki case the USG struck al-Awlaki down simply because he was a "dissenter". That's all.
What I do say is that a public servant should not have the right to declare someone worthy of death without any legal process and without any verifiable evidence. There are legal proceedings in absentia all the time. The protection of not having a trial in absentia, making it illegal, is so that the punishment is just. It is not a loop hole to say, "to hell with it let's just kill because law."
Is the evidence for literally all the claims above still "leaked to a newspaper by a public servant but officially neither confirmed nor denied"? Or are some public servants "on the record making the claim"? Is there anything testable and verifiable?
The claims may be entirely correct, I don't know and I don't actually care much. A claim is not evidence. I claim there are war criminals in the US government right now who should be extradited to the Hague, but so what if I claim that? It's of mild interest to some but has no baring on whether shooting them is legal, justified or something I should do anything but oppose.
Your closing comment:
"What I don't think you can say is that in the al-Awlaki case the USG struck al-Awlaki down simply because he was a "dissenter". That's all"
Strawman. Nobody claimed that in this conversation.
Does this mean you would have supported the right of Iraq to use drone strikes against the pentagon before Iraq capitulated? It certainly is a military target, so presumably that would be ok, after the invasion started? (I'm not asking if you'd be happy about it, but asking if you think it would be reasonable and fair according to international law).
Thomas has already covered this quite well, but I'll briefly summarize again. al-Awlaki was actively at war against the USG. Killing him is roughly equivalent to killing US citizens who fight for other foreign governments. The drone strike was not carrying out a legal sentence, it was simply the use of force in a global conflict.