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.