This wasn't a city per se, but I've done research regarding the sack of 圆明园 (The Old Summer Palace / Gardens of Perfect Brightness -- not to be confused with much more famous 颐和园 Summer Palace) by predominantly French and English forces in 1860.

There is extensive detail found in journals and letters written by a number of the soldiers and the English commander leading the expedition, Lord Elgin the 8th. Lord Elgin wrote:

"Plundering and devastating a place like this is bad enough, but what is much worse is the waste and breakage. Out of 1,000,000l. worth of property, I daresay 50,000l. will not be realized. French soldiers were destroying in every way the most beautiful silks, breaking the jade ornaments and porcelain, etc. War is a hateful business. The more one sees of it, the more one detests it."

Looking at these journals, no one specifically regrets looting, but there is remorse for the damage, destruction, and waste. To the best of my knowledge, there weren't reports of raping.

If your curious, the sacking of the palace was not for total war purposes but retribution for their deceased comrades, killed as POWs. Also, the emperors reluctance to sign an unfair treaty played a significant portion.

As with most things related to the moral standing of pre-modern people, this is very hard to assess, as there is an incredible amount of factors to consider, and even then, there are ethical notions that we can barely conceive as such, judging from our perspective.

Speaking about the Middle Ages, I can say that the abuse of civilians was a big concern for all involved, especially the church. It was definitely viewed as quite a bad thing, but they also took into account the realities of warfare, which made pillaging a necessity. The result was the doctrine of just war, developed by Saint Augustine, Thomas Aquinas and many other relevant figures, which stressed the need to excercise force just in the amount that was necessary. Regarding this, it's important to know that medieval commanders and theorists realized that war is not a conflict between armies, but one between nations, and as such, involved the people.

All of that being said, most noblemen commanding the armies attempted to limit the brutality to what was really necessary to cause a strategic impact (burning crops, sacking manufactured goods, and in some circumstances, doing some killing), but rape for instance, was quite frowned upon and, commanders often tried to avoid it. The result of this, is that you would see knights leading chevauchées, but hardly partaking in them, much less in the more brutal parts of it.

Theere was also a very complex set of norms regarding prisoners of war. It had been believed until recently that only noblemen were captured for the purpose of obtaining ransome, but recent evidence suggests that there was a large and well organized market for ransoming even normal soldiers. On the other hand, the execution of POWs did definitely occur in some occasions, particularly when the enemy was perceived as alien (which is to say, a person not belonging to the civilization of Latin Christendom, which includes heretics, muslims, and people from the catholic periphery, like certain Slavs). But even then, there were often moral qualms about executing certain prisoners. Here is what Raymond the Great had to say during the conquest of Ireland in 1170, when his troops captured 70 enemy soldiers:

"Had we killed them in battle, it would have increased our prestige; but since the moment we made them prisoners, they are no longer enemies, but human beings. On the other hand, they are not rebels, nor traitors, nor thieves, but men who were vanquished defending their homeland. Let us then be merciful, for mercy is worthy of praise. Without it, victory is bad, brutal. Also, their ransome will be of more benefit to us than their death, for it will allow us to increase the wages of our men, and give an example of noble behaviour". 1: Gillingham, J., "Conquering the Barbarians: war and chivalry in twelfth-century Britain", Haskins Society Journal, 4, 1992, pp. 68-84.

So as you can see, this whole matter is extremely nuanced and it's hard to draw over-arching conclusions.

My sources, among others that I don't have at hand to cite, are:

Flori, Jean, Knights and Chivalry in the Middle Ages.

Keen, Maurice, Medieval Warfare: A History.

The studies that I mentioned as pertaining to prisoner exchange are the research of Dr. Rémy Ambühl, from Southampton University.

EDIT: to correct typos.

In the Hundred Years War, circa 1380, soldiers would be expected to provide for their own expenses out of their own money- food, clothing,weapons, horses, etc. They would get a recruitment bonus, and a modest wage, but they would expect to also be able to pillage. And as this expectation made it easier to recruit soldiers at a lower wage, the captains who were charged with raising armies would not quarrel with it. When John of Gaunt teamed up with the Portugese John of Avis against the Castilian John of Trastamara, there was an actual agreement made by the two about sacking Valderas. The English were to have a head start... but the Portugese soldiers jumped in anyway.

If they sound like criminals, well, they were. Professional soldiers would often become routiers when unemployed, forming robber bands, taking fortresses, and using them as bases of operations for exacting tribute from the locals, or kidnapping them for ransom, ranging far afield and attacking other towns. And when peace came, men arrested for murder, theft, housebreaking, etc. were not uncommonly ex-soldiers. If they were lucky they'd served under somebody important , who had influence, and they'd be pardoned. If not, they'd be executed.

Jonathan Sumption: Divided Houses