Having lived among that culture for a while I would disagree. As difficult as it is from our cultural perspective, there are people (not all) who have no qualms about the taking of human life especially if there is great hatred-not only in Islam but here in the United States. But do these people feel ANY type of moral conscience that is just surpressed? If so it is so far surpress that it isn't showing even to them. They're just serial killers who kill and want to kill again.
If everybody's value system was somewhat consistent then natural law could possibly be argued. Unfortunately, people's value systems are NOT consistent. Different cultures have different rights and wrongs from which their laws are based. Consider Sodom:
Gen 19:9 But they said: Get thee back thither. And again: Thou camest in, said they, as a stranger, was it to be a judge? therefore we will afflict thee more than them. And they pressed very violently upon Lot: and they were even at the point of breaking open the doors.
They didn't like anyone telling them what to do. They looked at Lot as being judgmental. For us to say that, "Oh, they really KNEW what they were doing was wrong but it was just surpress." is kind of crazy in my mind. There was no talk about considering Lot's proposal. They knew precisely what they were doing.
The more I think about natural law, the less I agree with the concept.
But won't these same murderers resist being murdered? They'll treat their own murder as wrong in principle, even if they don't treat others' murders that way. Their resistance suggests they are conscious of the wrongness of murder, even if they don't apply that principle consistenly.
I think you might be underestimating the human capacity for moral knowledge. To paraphrase Augustine, even a gang of pirates has a certain sort of justice--otherwise their gang would dissolve in an orgy of self-destruction.
Your point about hatred gives a bit of insight into how a natural law theorist might respond to your critique. Let's suppose for the sake of argument that there is a natural law against murder which can be captured in the statement: "the direct taking of innocent human life is always wrong." Now because of extreme hatred or bad philosophy or bad religion, Jihad Joe thinks it's OK to crash planes into American skyscrapers. Why does he think so? It's not because he thinks taking innocent life is good. He thinks it's good because in his mind all Americans are evil, enemies of Allah, supporters of Israel. Read bin-Laden's statements, he doesn't say "let's kill off as many innocent people as we can," in his view Americans are collaborators with Satan who should be either destroyed or converted to Islam.
Some people here seem to think that Natural law theorists believe that human reason is infallible and always right, such that humans' innate understanding of the natural law is always rightly applied. This is far more than any such theorist would claim. Human reason can be clouded by sin, runaway emotions, and error. That doesn't mean there are no self-evident principles of moral reasoning.