While I disagreed with the substance of your post, it was the form that most annoyed me. (You could say I should focus on the substance, and you'd be right, but... I didn't.) Saying "theory X (that most people don't agree with) says Y about this subject, which supports my position" isn't a very effective argument unless you give reason why people should believe or trust X. As I said, I would hopefully have been less annoyed by it, except that I just ran into the same form on a completely different topic.

[1] Here I am presuming that the doctrine of double effect does not absolutely and automatically follow from natural law. I suspect that natural law is not unambiguous enough for that. I am not an expert, though; feel free to disagree if you think I'm wrong.

I myself do not accept the doctrine of double effect. When faced with a "do I or don't I" decision, inaction is just as much a choice as action, and you're also responsible for the (foreseeable) consequences of the inaction. If Truman had not dropped the bomb, and the US had not invaded, Truman would have borne some responsibility for the continued deaths in China, which his inaction allowed to continue.

Or, to use a different example, if you see the crime happening and you do nothing, you are to some degree complicit. You're not in the same category as the person who commits the crime, but you still have some responsibility.

(Hey, look, I'm giving unsupported assertions too! It's really hard not to, because nobody's got time to go back to first principles, agree on them, and then derive conclusions.)