First of all it wasn't athens. It was rhodes.
In God of War, Kratos got himself nearly killed by the Barbarian King because he wanted to spread the glory of Sparta, that was his ultimate mission. He was promised his memories cleaned and was promised peace in exchange of Ares' death. However in the end, the gods say that what he had done was horrible and cannot be cleaned. Therefore he is pissed at the gods for breaking their promise and pissed at himself for the things he did.
His only joy comes in the form of the "glory of sparta" which he intended to achieve when he was a mortal. So he helps his native people to spread its glory..simple as that.
Zeus who is afraid that Kratos would one day, try to be the king of the gods by overturning his rule, just as to he himslef did to Kronos, out of fear - kills kratos..who comes back from the dead seeking vengeance as it was because of the gods that he lost his family and he get those nightmares about himself, and they stabbed him in the back after he had done everything they had asked for.







