Maybe. This is federal actiom. Killing is deprivation of life. US citizens are generally construed to have due process rights anywhere in the world. So due process clause applies. The question, then, is what process is due? The clause means exactly what is said: people are entitled to due process, not more process than is due. Due process doesn't always mean a trial. It's a sliding scale. The more something looks like something that would require a trial, the more process is due.

What process is due when you go to Yemen and take up arms against the US? Should the government have to apprehend you abroad and bring you in for trial?

If it sounds fuzzy its because it is. The framers used a wiggle word like "due" to give interpretive leeway. If they had meant hard and fast judicial process they would have said so.