Why did Homer’s Greeks sack so many cities in the Trojan War? Does it make them mostly pirates and slave raiders?

The Achaeans and others sacked many hillforts because "state of Troy" was an area of about 450 hillforts.

Also, the conquerors were frustrated and hungry because the war persisted for 10 years (I am talking about theory – Troy is in Istria).

Many of the hillforts were placed near the rivers and sea gulfs. The city of Ismaros you could recognize in the northern part of Istria, in Slovenian city Šmarje (Ismaros = I-SMAR-OS = Smar-ios = Šmarje).

Also, the Kikonians have today's toponym-village Kikovija in the highlands in the east of Istria. Achilles was angry at Agamemnon, and he wanted his part.

Another problem was: they had to bring something to their homes after ten years of war.

Some critics say Homer's story is not exactly history (because location of Hisarlik is not adequately), but when you read Homer with vision of Istrian peninsula, everything is in harmony.

One important fact: It is not a story of the Greeks; Homer never used the word "Greeks". The Achaeans came from Dalmatia.

Think Bronze Age Collapse. The entire civilized world of the late Bronze Age went down in about 1077 BCE. Of the existing civilizations (Hittites, Assyrians, Canaan, Ugarit, Sidon, Tyre, etc,) all collapsed and only the Egyptians did not. They nevertheless fought the "Sea Peoples".

It starts with droughts and fires followed by earthquakes. Displaced seagoing people become marauders. They head off looking for cities to sack, since their own City States are in chaos and can't sustain them.

Cities that collapsed in Anatolia: Troy, Miletus, Hattusa, Mersin, Tarḫuntašša.

In Cyprus: Palaeokastro, Kition, Sinda, Enkomi.

In Syria: Ugarit, Tell Sukas, Kadesh, Qatna, Hama, Alalakh, Aleppo, Emar.

And, in case you were thinking that it was all right back home for Agamemnon's followers, these Greek cities show evidence of collapse: Teichos Dymaion (el), Pylos, Nichoria, Menelaion, Tiryns, Mycenae, Thebes, Lefkandi, Iolkos, Knossos, Kydonia.

My source for the list of cities above is: https://en.wikipedia.org/wiki/Late_Bronze_Age_collapse

You mention correctly that what is described in Homer is not exactly history. But to answer this question we may suppose it is.

The general principle is that when seriously discussing history one should not impose the modern labels, and discuss it in the terms and notions of the time when it happened. Yes, they were "slave traders" in the sense that they captured slaves and traded (exchanged) them. But at that time, this was a completely normal activity. And in many much later times, and in many other cultures. Same applies to sacking cities and killing all inhabitants. This was just a normal practice for many thousands years everywhere. Of which we have abundant evidence in the literature. In the Bible, for example.

The society notions on what is good and bad, and what is admissible change with time.

Why did Homer’s Greeks sack so many cities in the Trojan War?

"Why" questions can have many different answers that involve different modes of explanation. LamaDelRay's answer gives an explanation mainly in terms of the culture's religious beliefs, which clearly makes a lot of sense if you take Homer as a description of how people really thought the universe worked.

A whole different mode of explanation would be game theory and biology. In terms of game theory, it's risky to carry out violence against another group unless you wipe them all out. If you leave some of them alive, even the children, then they may take revenge on you later. Biologically, evolution selects for behaviors that enhance reproductive success. Kidnapping the women and using them as sex slaves could clearly serve this purpose. None of this is nice, and it doesn't provide a moral justification, nor do these explanations require that the actors consciously understand them. (Chimpanzees display similar behavior, and they probably don't understand why they do what they do.)

By the way, Steven Pinker's book The Better Angels of our Nature has a lengthy discussion of Homer on pp. 4-6. In it, he talks about the religious factors pointed out by LamaDelRay. Elsewhere in the book he gives the game theory and biological explanations for genocide and mass rape. (His main point in the discussion of ancient cultures seems to be to attack the myth that total war and genocide are a modern phenomenon, or that modern war is more deadly than war was in the ancient world.)

Really, the values of that time were pretty different. Every Greek in the war is a bloodthirsty warrior aiming for glory. They all know that gods are likely to bless or curse them if they do something, this is what they call "fate". Even if they are Greeks fighting Greeks, the others that died by their hands "deserved" it, it was the Gods' agendas. Or else they would be cursed afterwards...

So Homer's portrayal of the Greeks as warmongers is pretty much normal. After all, Athena might come to help the innocent bystanders if they are truly innocent, so why bother? It's much better to follow your urges than repress them, at least, in this early Greek portrayal.

Also, "Greece" was not united. Every city could become an enormous empire by invading the others, they weren't really close to each other. This is quite similar to early China (before Qin united everything.). They think they belong to the same cultural core, but each of these is comprised of a subdivision of the Hellenic culture. That's also why life in Sparta was different than life in Athens.

Let's simply say that Homer didn't wish to show Greeks as pirates but more as badasses that are totally able to wreck whatever stands in the way of their desires.

You're looking at this with 21st century eyes. Back then, pillaging and looting were part of war. It was customary to let your troops plunder for a few days (typically three) after winning a siege – doing so basically was how you'd pay your troops.

A few examples of atrocities that shocked by the standards of the time:

• After the Romans defeated Carthage, they sold its population into slavery, and burnt and razed it to the ground. The event was unusual in that the Romans additionally salted the grounds for good measure.

• After the Crusaders captured Jerusalem for the first time, they massacred the Muslim and Jewish population wholesale, leaving few survivors behind.

• The Sack of Constantinople during the Fourth Crusade went down in history as one that a 20th century author described as having been "on a scale which even the ancient Vandals and Goths would have found unbelievable".

• The Mongols practiced the "promise" of wholesale execution, which is best described as a form of psychological warfare: they'd massacre every last person in cities that would resist them, bar a few survivors who were then let loose so they could spread the word that resistance should not be an option.

• During the Franco-Dutch War of 1672-1678, Turenne shocked Europe in 1674 by plundering the Palatinate far and wide, essentially burning the whole region to the ground, in an scorched earth effort to cause logistics-related problems to Imperial troops arriving as reinforcements.

At the other extreme, history also tells us of milder sacks and unusually generous surrender terms:

• When the Visigoths sacked Rome, it shocked contemporaries but it was actually restrained by the standards of the time: there was no general slaughter of the inhabitants, the two main basilicas of Peter and Paul were nominated places of sanctuary, and most of the city survived intact – though stripped of its valuables.

• When the Vandals sacked Rome a few years later, they were a bit more thorough – staying 14 days vs three, with a bit more damage done. But Pope Leo I had persuaded Genseric to not raze the city or kill the population wholesale – certainly a welcome concession.

• Saladin, in stark contrast with the First Crusaders, offered the Christians surprisingly generous terms by the time's standards when he reconquered Jerusalem. He went as far as allowing many families who could not afford their ransom to leave, against the wishes of his treasurers.

Mindsets and the standards of troop behavior eventually evolved. Restraint was expected by the early 20th century. To illustrate this, consider how Kaiser Wilhelm II sent his German troops to China as part of the punitive expedition against the Boxer Rebellion: he instructed them to show no restraints and basically 'behave like Huns'. The anecdote is particularly telling, but not because it shocked some at the time or because it was used a few years later for WW1 propaganda purposes. Rather, because a few centuries earlier it would have been so commonplace to 'behave like Huns' that it would not have been worth mentioning. Times had changed. Times had changed.