Right. The Greek mind has a very odd relationship with altruism. It might be easier to explain the mentality in terms of cosmic order.
It's no coincidence that the Greek creation story ends - and history begins - when anthropomorphic gods win a great war with the forces of primordial chaos. They exile some and adopt others, but the Titanomachy is in many ways a story about human-like beings bringing order and civilisation to a wild and untamed cosmos. The mortal world tends to reflect the divine world in a similar way: humans bring order and civilisation to a wild and untamed nature. The allegories are usually pretty obvious. Zeus, the warrior-king, father-ruler, and head of the household enforces this order, in the same way that the Greek father ruled his
oikos (hence the occasional title of
oikodespotes), which in turn was the way that the king or tyrant ruled his
polis. Zeus' authority, like the father's authority and the king's authority, doesn't rest on being a nice person, but on two separate axes: his ability to rule and defend his own, and his ability to enforce stability on a fundamentally chaotic background.
Or in other words, if you're a Greek commoner, there are two reasons the king or tyrant should stay in power. The first is that he's really good at fighting, or can call up a bunch of guys who are really good at fighting. So he can beat you up if you don't like him, but more importantly, he can beat up anyone from outside who comes along and tries to push you around. (For remember that most Greek city-states were near-constantly raiding one another.) The second reason is that through his monopoly on force and through defining (mostly) consistent laws, he makes civilisation possible. If everyone is out for themselves with no constraints on their behaviour, things are terrible for everyone. (Thus also the immense importance of things like honour, oathkeeping, civic responsibility, and so on.)
All of this is bound up with an ordered view of the cosmos. The Greeks were self-consciously civilised, and you will notice in their mythology that the most common villains are either forces of nature or barbarism. They often stand out for their wantonness and their lack of that all-important Greek virtue: self-control. Hence the large number of theriomorphic villains: the minotaur, centaurs, and so on. (And note how the centaurs were often depicted as practically the ancient equivalent of a bikie gang: they raided towns, got stinking drunk, raped women, and so on.) The Greeks loved writing about monsters for the same reason. The hydra, the Nemean lion, the Stymphalian birds, the chimaera, the list goes on. (The Roman tradition of beast hunts is similar. Why did the Romans so love seeing trained gladiators kill exotic monsters in the arena? Because it demonstrates their mastery over the chaotic, primordial forces of nature.) Human villains are similarly barbarous. The Amazons are a good example; they pervert the cosmic order the Hellenic city-states represented by inverting gender relations. Other
barbaroiare pretty much as 100thlurker states. Without a city-state - without an
ordered community - they're barely even people.
When this tangent got started, I talked a lot about Greek heroism being selfish. That's not
preciselytrue, but it is worth emphasising that Greek heroes are not altruistic in the same way we think most modern heroes should be. (There's actually a huge amount of variety in modern heroes as well, but - I think deriving from Abrahamic traditions - there's almost always the assumption that a true hero is selfless and compassionate.) Greek heroes have a very strong sense of civic responsibility. The community is order. The
polis is what gives you your identity. You can change
polis, but you can't abandon the way of life it represents without becoming a slave or a barbarian or an animal. Greek heroes thus value their social standing very highly. You'll notice in a lot of Greek myths that the heroes go out of their way to respect mechanisms of social status or reputation.
(Examples: It never even seems to occur to Herakles that he could just tells Eurystheus of Tiryns to stuff off and not do the Twelve Labours. The labours are an act of public penance for murdering his family. Their completion and formal absolution by the monarch is a requirement for his participation in society. Or for another example, after Medea murdered Absyrtus, Jason is compelled to visit Circe in order to be publicly absolved of guilt. Antigone, in the play of the same name, is motivated by concern for the honour and reputation of her family. There are lots more.)
Explaining the difference between this behaviour and what we might think of as modern or Christian altruism isn't easy, but I would speculate that it's because the Christian tradition always had a stronger counter-cultural aspect. So did Judaism, to be fair, and it is striking how counter-cultural most of the Hebrew scriptures are. The prophets are speaking
againstIsrael more often than not, and when you filter in the huge amount of exilic literature, that tradition is often speaking from a perspective of oppression. Meanwhile Christianity's early years are marked by a struggle for identity, against the background of surrounding pagan cultures. There were no Christian cities or nations during its formative years; just small circles of believers trying to stick together. Therefore one of the dominant themes of early Christian literature (notably most of the NT epistles) is the need for Christians to support and love one another, in order to survive. "Please don't fight. Try and forgive one another. Be peaceful." While eventually the great heresies arose and Christians took more aggressive, even martial approaches, the key formative years of the tradition gave it this more low-key approach. And of course from the Jewish background you got a tendency to downplay the individual, and to embrace humility before God.
So how can I put this... in the Judeo-Christian tradition there's a deep-seated suspicion of the powerful, rooted in experiences of oppression. Compare what I just said about the Greek ruler on the model of Zeus, whether he be king or tyrant, to biblical
suspicion of kings. Israel paid tribute to and was conquered by foreign kings often enough that they were more inclined to view militarily powerful and glorious figures as dangerous. You
can't trust powerful people. A scholar I know recently described the OT to me as an extended cosmic protest against chaos, and an affirmation of order in the midst of it, but where the Greeks saw cosmic order reflected and embodied in political order, the Judeo-Christian tendency was to see order as something a bit more subtle and subversive. For them God was more likely to work against states than through them. It's a bit of an inversion, really, right up to the early Christians ironically crowning an executed barefoot preacher 'King of the Jews'. On the whole, the point is that the dominant Greek cultural experience of the warrior-hero was a positive one. He defended the community, founded the city, and brought order. The Judeo-Christian experience was quite different. There are certainly Jewish warrior-heroes (Joshua, Gideon, David, etc.), but I think they were on the whole much more hesitant to make them the highest objects of praise. Not after the fall of Jerusalem, at any rate.
Arguably, then, Greek mythology tends to take the perspective of the strong, while Judeo-Christian thought tended to take the perspective of the weak. (That occasionally ended up in very weird places, once Christians had
become the strong. See: Christian justifications for feudal hierarchy, or increasingly convoluted attempts to pretend that colonialism is done for the sake of the conquered.) The Christian shift had to do with humility, the need for reciprocal charity and love, and a mild paranoia about people with power.