Was Awlaki someone I'd want to ever talk with, or even meet? No. But he did have rights as a US citizen (and, one would hope, just as a person), and the most important of those rights is the right to live.
Did he commit any crimes? Not that we're aware of. But even if he did, he still has the right to a fair trial to establish that he did, in fact, commit those crimes. And then to be sentenced by a jury of his peers.
Did he say some terrible things? Well, I haven't heard any of his speeches, but from what I've heard second- or third-hand, he probably did. Is that illegal? It is not. Free speech is not just about protecting things that most people want to hear. In fact, it is not about protecting those things at all; they don't need to be protected. It's about protecting those on the fringe. And the courts have established, over the years, that pretty much anything short of incitement to imminent violence is protected. So he certainly cannot (legally) be killed over what he was saying.
So we have a very clear case of abrogation of justice (at the very least), and a President getting the ultimate in tyrannical power: the power to kill your political enemies. Let's hear it for justice.
Update: I forgot to make note that the US official reporting this is granted anonymity; this is also very disturbing. And the official apparently claimed that al-Awlaki is
suspected of inspiring or helping plan numerous attacks on the United States
I'm skeptical of this, if for no other reason than we didn't hear anything about this before his killing. But even if we grant it as true, suspicion does not justify killing. You need to be 100% certain that he did, in fact, do those things, before issuing a death sentence. And you need to demonstrate it clearly enough that a jury feels the same way, and gives that sentence. None of that ever happened.