Sorry, but that is just wrong! No American whatever his power has the sanction to condemn another American to death.
You really need to look and the bigger picture...
Do you really want the President to have a hit list and the power to put any American to death on his word? I would think not, but if you really believe what you are saying, that would be the outcome. It does not require the Congress or anyone else to sanctify or disagree with the President, he would have the power? To allow that would be the start of another revolution...
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
I see two violations here: Anwar al-Awlaki was (clearly) a traitor to the United States, a capital crime, executed without the presentment of an indictment. He was deprived of life, liberty, or property, without due process. If a FISA Court -- or even a military tribunal -- had found him guilty of waging war against the United States, there would be no issue. But the President of the United States is not part of "due process."
He was killed in a country we are not at war with, so the President's war making power has dubious applicability. Bin Laden, and 99.999% of the other terrorists aren't entitled to Constitutional protections. Unfortunately, the two Americans killed in Yemen were.
I am very happy he's dead. I'm very happy the other American is dead, too. But you cannot possibly believe that it's a good idea to give Barack 0bama the authority to kill a US citizen on his own authority. It isn't.