I'm not even so concerned with him being a pawn as I am both the precedent it would set (government can kill people based on unproven claim they're a "terrorist" - the most malleable and abused word of the last decade) and the impact it would have in the Middle East. If, as all available evidence indicates, he's just a spokesman and advocate rather than engaged in terrorism, that would open our targets to thousands of Muslims around the world who preach against the US and would surely result in both greater radicalization of the general populace and 10 new al-Awlaki's ready to take his place.
Ultimately, anything we do outside the rule of law only exacerbates our problems.