The constitution clearly limits the powers of the government. The 5th amendment clearly states that people shall not be deprived of their lives without due process.
The way I see it he had repeatedly called for jihad against all Americans which if he considered himself one would have meant jihad against him so he had already, by 'de facto', renounced his citizenship. Besides it was the use of an international law that the administration used to target him as a terrorist.
And people wonder why many are against international laws trumping the constitution.
There are circumstances where Actions are protected under the Constitution and take precedence, or are exempt. Treaties are only one example. War Powers are another.
I was trying to keep it simple if you catch my drift.......