I think you are confused.
These are not people killed on a battlefield, but in their homes.
I do remember the Constitution saying quite a bit about due process for citizens.
These were (or at least were claimed to be) enemies of the United States at active war with us. They were attacked in a foreign country under the terms of a congressional authorization to use force, the functional equivalent of a Declaration of War.
Is there some law of war of provision of the Constitution of which I’m not aware whereby enemies aren’t to be attacked in their homes?
As I said, I have no way of determining whether these were legal targets, but if they were what do you you think the procedure for attacking them should be?
For some obscure reason, people get all bent out of shape by the use of drones. But a drone attack is no different in principle from a fighter strike, a commando attack like the one that killed Osama, or a sniper attack. If any of those are justified, then so is use of a drone.
The Constitution does not specify due process for “citizens.” It specifies, in both 5A and 14A, that “no person” is to be deprived of life, liberty or property without due process.
Since military action is by definition the opposite of due process, the condition obviously does not apply.
As I’ve said before, whether a given military strike is proper and legal is an entirely different question.