The implications of killing an American citizen without even a trial are scary though.
If you are stating your personal view on the subject, then I understand and you're certainly entitled to it.
From a legal standpoint, the fact that Al-Aulaqi was targeted for taking operational part in groups engaged in armed conflict with the United States seals his fate, and his citizenship in the U.S. affords him no more due process rights than his terrorist brethren.
If you are more (subtly) commenting on the executive branch's summary decision-making that he was an enemy combatant, it's worth noting that the CIA and the U.S. gov't does not take the position that it's entitled to "summarily execute" *any* citizen-enemy/combatant. Rather, it's internal position seems to be that, as a matter of policy and as authorized under the Authorzed Use of Military Force, it could execute Al-Aulaqi because he:
1.is believed to be “part of” enemy forces within the meaning of the AUMF;
2.has been on notice for a lengthy period of time that he is regarded as such, is clearly aware of that, and has not only not denied it but actively taunted U.S. forces about their inability to get him;
3.has not made any attempt to surrender;
4.is believed to be playing an active, operational role in attacks against the United States; and
5.is camped out in a country that is unable to exercise civilian authority in the region in which he is located.
Based on these justifiable findings, substantiated at that, I find this military action far from "scary". (Heck, you might not even need the AUMF, and could simply justify the action under the President's power as Commander in Chief under Article II).
But again everyone's entitled to his or her opinion.