This is a simple but profound Principle of Natural Law. Willful deception for the purpose of manipulating the actions of another person is a clear and egregious violation of Natural law. The reason it is a violation of Natural Law should be clear: willful deception is an attack on a person’s free will.
First, Natural Law dictates that every individual is personally responsible for their own actions. This means every individual has a duty to make sure they act within the confines of Natural Law.
Natural Law dictates that no one has the authority to trespass against the Natural Rights of another individual, or against the Social Contract which forms Society. This means every individual has a duty toward both Society and every other individual with whom they interact. They have a duty to safeguard their rights and uphold the principles of Natural Law.
This duty toward Society and other individuals creates a secondary duty to be informed. A person must know and understand Natural Law so that they know and understand what is and is not a Natural Right, and what is and is not a violation of those Rights. They must also know and understand Natural Law so they know the limits of a just government.
In the case of individual interactions, a person must know and understand Natural Law so they can apply it to the governance of their private affairs. This means they have a duty to learn as much as they can about the individual(s) with whom they are dealing, as well as whatever deal of interaction into which they are entering.
In the case of Society, a free and self-governing people have an additional duty to be informed about any issue on which they might vote or otherwise influence the Social Contract. It is not enough to know and understand Natural Law; a citizen must also know the issues well enough to properly apply Natural Law to the governance of their participation in self-government.
In both cases, anyone who knowingly and willingly lies or otherwise acts to deceive or conceal information is guilty of violating Natural Law. In the case of individual interactions, deception trespasses against the individual’s right to be fully informed so that they can make proper decisions for their personal affairs. In the case of Society, deception not only tramples on the individual’s right, but also on the right of Society as a whole under the terms of the Social Contract. In both cases, deception is fraud, and fraud is a clear violation of Natural Law.
Therefore, because deception is fraud, and fraud is a violation of Natural Law, deception is a violation of Natural Law.
If it helps, think of it this way:
If a person must deceive you to get you to do what they want you to do, they are admitting that what they want you to do is wrong and that — if they told the truth — you would not do it. Therefore, the very act of deceiving implies and admission of guilt by the party seeking to deceive.
If one person deceives another and tricks them into acting in a way that they otherwise wouldn’t have, then the person who is deceiving is placing a claim on the will of the person they deceived. This is a form of slavery, and slavery is a clear violation of Natural Law.
Any way we look at it, deception is a form of manipulation of another person’s actions, which is a trespass against their free will. This clearly demonstrates that willful deception for the purpose of manipulating the actions of another person is a clear violation of Natural Law.