You might want to take a look at Ashoka who first comes to my mind as possibly fitting what you seek.

Ashoka was the grandson of Chandragupta who founded the Mauryan empire which eventually included most of what is now modern India as well as substantial regions to the northwest. Although unfamiliar to most Americans (speaking as an American here), it was a VERY substantial empire.

In any case, Ashoka was supposedly a quite warlike emperor... right up to the conquest of the Kalinga region. The horrible suffering inflicted upon the local population is said to have filled Ashoka with horror and remorse. In fact, Ashoka wrote about this himself by having a huge number of carvings set up around his empire with his thoughts on how his people should live. These are the Edicts of Ashoka.

The one pertinent to your question is number 13. In part, it says (and note that King Piyadasi is a reference to Ashoka, not someone else):

Beloved-of-the-Gods, King Piyadasi, conquered the Kalingas eight years after his coronation.[25] One hundred and fifty thousand were deported, one hundred thousand were killed and many more died (from other causes). After the Kalingas had been conquered, Beloved-of-the-Gods came to feel a strong inclination towards the Dhamma, a love for the Dhamma and for instruction in Dhamma. Now Beloved-of-the-Gods feels deep remorse for having conquered the Kalingas. (Note that he spoke of himself in the third person here.)

Indeed, Beloved-of-the-Gods is deeply pained by the killing, dying and deportation that take place when an unconquered country is conquered. But Beloved-of-the-Gods is pained even more by this -- that Brahmans, ascetics, and householders of different religions who live in those countries, and who are respectful to superiors, to mother and father, to elders, and who behave properly and have strong loyalty towards friends, acquaintances, companions, relatives, servants and employees -- that they are injured, killed or separated from their loved ones. Even those who are not affected (by all this) suffer when they see friends, acquaintances, companions and relatives affected. These misfortunes befall all (as a result of war), and this pains Beloved-of-the-Gods.

If you want a bit more information about Ashoka, try this article from Encyclopedia Britannica.