Trust me, as a retired Army officer who spent a long time engaged in and teaching counterterrorism operations, I had no use for any of the three and I agree that they were traitors who needed to be at or below room temperature.
However, we are either a nation of laws or we aren’t. The President of the United States does not have the authority to order the assassination of anyone, especially a US citizen. Every American citizen, no matter how deplorable (and no, I’m not talking about we Trump supporters) is entitled to due process.
With respect to 8 U.S. Code § 1481 and INA § 349, the clause “with the intention of relinquishing United States nationality” is absolutely critical. Losing one’s citizenship does not only entail the performance of the actions mentioned in §s 1481 and 349, but performing them with the intent of relinquishing one’s citizenship.
I’m pretty sure joining ISIS is renouncing citizenship, but I’m no lawyer. I just think anyone who is actively supporting an unlawful belligerent force has explicitly renounced citizenship in fact.
I’m not aware of a specific precedent, but I imagine there were Americans who joined the Axis is WWII, and wouldn’t have received treatment on the field. Taking up arms against one’s country or supporting those engaged in an unlawful war seems pretty brazen to me.