First: We've taken terrorists alive all over the world. Second: there's a Constitutionally approved way to deal in countries where we have no police authority; it's called a Letter of Marque. Third: as I said, IF a FISA Court had issued an indictment and a warrant, I'd have had no problem with killing him when he refused to surrender to US protection. That refusal could have been implicit simply in his remaining a fugitive. There is a FISA ruling in his case, but it doesn't go far enough. The fact that there was a FISA opinion concerning him should tell you that the DOJ was not completely happy with the fact of killing him -- even outside of New Jersey.
The fact that we keep prisoners outside of US jurisdiction who are not American citizens -- precisely because we do not want them to fall under the heading of "US Persons" should tell you that US Constitutional protections are taken seriously, even by nominally conservative Presidents.
The Constitution isn't a technicality, and this is not a fine point. You want to grant the President this kind of power, OK. The Founders didn't. And I don't either.
Do you honestly think he would have voluntarily surrendered to the authorities to be extradited to the US?
Look, I appreciate your concern for the constitutionality of the operation, but I think you are missing something somewhere. I recall something about German terrorists who were US citizens actually living in the US during WWII, and my understanding is that SCOTUS agreed that they could be tried as enemy combatants in a military tribunal. If that’s allowed by the Constitution, then I suspect that flushing this piece of crud down the toilet is too.