There is a theory that the names of major Greek Gods and Goddesses of the Hellenic Period were orginally the titles of the Kings and Queen oif specific tribes. Thus Zeus represented the rulers of heridng and farming communities and is associated with the sky, lightning etc because their primary concern was the weather. The equivalent among coastal dwellers was Poseidon while Hades was the title of those who ruled communities that specialised in quarrying and mining. As time went on, of course, things changed and the titles of Kings became the names of Gods - probably around the time that said Kings decided they were no longer prepared to be murdered at the end of their one-year reigns.

At any rate, it seems that the economic and and military superiority of the agricultural tribes led to their political dominance and thus the dominance of their god. But pagan religions as a whole are less exclusive than Abrahamic ones, and have the habit of absorbing and reconciling different cults into their own structure, rather than suppressing them. Thus, when the Olympian cult became dominant, Poseidon and Hades were demoted to being Zeus' brothers, assigned by him to manage the Sea and Underworld respectively.

Later still, when crafting and smithing became more specialised and economically important, many of the functions of Hades were assigned to Hephaistos, while Hades took over most of the functions of Thanatos, becoming the ruler of the dead while Thanatos was reduced to becoming deaths' personification.

As to the whole 'Gods Behaving Badly' thing - that might well have come about as a reaction to the unpredictability of Nature. Something which prehistoric people might well have been inclined to put down to the malice, mischief and quarrels of the spirits they believed inhabited everything.