Con men and abusers use deceit to exploit the weak. Why engage in it if your intentions are not exploitative? Deceit may be used in all parts of our life, but we refrain from doing so in most cases. There is no rationale for using deceit beyond manipulating people for our own benefit. But consider Sun Tzu’s viewpoint in the art of war.
The worst type of warfare is a direct, protracted conflict. This is where two equal powers declare their intentions and both armies fight head on. Since the forces are equal on both sides, the conflict becomes long, costly, and bloody. If deceit can be used to avoid direct conflict, then it is ethical to use it because it reduces harm.
Imagine a fictional scenario where the ruler of one empire (Wu) understands that a war with the neighboring empire (Tzei) is inevitable. Conquering Wu would provide Tzei with vital resources and trade routes. All previous truces and trade agreements will not realistically prevent this war. The ruler of Wu predicts that Tzei could immediately deploy an army of equal size to that of Wu, but the Tzei ruler would prefer to buy time and build up its army further.
The ruler of Wu then deceives Tzei by concealing his intention to strike at Tzei first. The Wu ambassadors to Tzei give contradictory messages of peace and deterrence, thereby creating the impression that the Wu ruler is confused, possibly frightened and indecisive. At the same time, the secret agents of Wu find a Tzei spy working in the Wu courts. Instead of arresting the mole, they feed him false information that will convince Tzei that the Wu ruler desperately wants to avoid a war. This convinces the ruler of Tzei to let down his guard and built up an army instead of attacking immediately. The invasion of Tzei by Wu forces a week later surprises the Tzei ruler and generals so much that they can not prepare an adequate defense in time. What could have been years of warfare is resolved within a matter of months by the defeat of Tzei.
Notice that in this scenario, Wu was acting in self-defense, which may mitigate morally the use of deceit and warfare. But consider if Tzei would have used the tactics of Wu instead, not for self-preservation but for personal gain of resources and trade routes. The intentions are less pure, but more importantly, a long direct battle was avoided, which would have entailed more costs and loss of life.
Do not delight in your cunning, but make use of it, because it is not possible to survive without it. Mask your true nature and conceal your intentions if this information can be used against you and what is dear to you.