A wheel turning monarch is an Ariya who is a king, an Ariyan King. A king must defend his nation by having an army, which inevitably results in killing. Even the Buddha said when there’s a strong king, thieves and bandits hide, but when there’s a weak king, good people hide and thieves and bandits are out and about.
Isn’t it against the first precept to not only kill, but to tell others to kill? How can an Ariyan King break that rule? or are Kings exempt from that rule because of their karma?
“Bhikkhus, even a wheel-turning monarch, a righteous king who rules by the Dhamma, does not turn the wheel without a king above him.”
When this was said, a certain bhikkhu said to the Blessed One: “But, Bhante, who is the king above a wheel-turning monarch, a righteous king who rules by the Dhamma?”
“It is the Dhamma, bhikkhu,” the Blessed One said. “Here, bhikkhu, a wheel-turning monarch, a righteous king who rules by the Dhamma, relying just on the Dhamma, honoring, respecting, and venerating the Dhamma, taking the Dhamma as his standard, banner, and authority, provides righteous protection, shelter, and guard for the people in his court. Again, a wheel-turning monarch, a righteous king who rules by the Dhamma, relying just on the Dhamma, honoring, respecting, and venerating the Dhamma, taking the Dhamma as his standard, banner, and authority, provides righteous protection, shelter, and guard for his khattiya vassals, his army, brahmins and householders, the people of town and countryside, ascetics and brahmins, and the animals and birds. Having provided such righteous protection, shelter, and guard for all these beings, that wheel-turning monarch, a righteous king who rules by the Dhamma, turns the wheel solely through the Dhamma, a wheel that cannot be turned back by any hostile human being.
https://suttacentral.net/an3.14/en/bodhi
Once upon a time, mendicants, there was a king named Daḷhanemi who was a wheel-turning monarch, a just and principled king. His dominion extended to all four sides, he achieved stability in the country, and he possessed the seven treasures. He had the following seven treasures: the wheel, the elephant, the horse, the jewel, the woman, the treasurer, and the counselor as the seventh treasure. He had over a thousand sons who were valiant and heroic, crushing the armies of his enemies. After conquering this land girt by sea, he reigned by principle, without rod or sword.
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‘But sire, what are the noble duties of a wheel-turning monarch?’
‘Well then, my dear, relying only on principle—honoring, respecting, and venerating principle, having principle as your flag, banner, and authority—provide just protection and security for your court, troops, aristocrats, vassals, brahmins and householders, people of town and country, ascetics and brahmins, beasts and birds. Do not let injustice prevail in the realm. Pay money to the penniless in the realm.