Hades was the god of the underworld. He collected the souls of the dead and put them in their appropriate places ranging from the Elysian Fields to ironic manual labour punishments. But here’s the thing. He’s not evil. He doesn’t have to buy souls, he gets them all at the end. Most of the time, the mythology actually portrays him as fairly meek and businesslike. The only exception is when he kidnaps Persephone.
He does have a lawyer-like appreciation of a bargain though. When Orpheus goes to retrieve his dead wife, he’s told he cannot look back until he is out of the underworld. On the threshold, he can’t resist a peek and his wife is sucked back into the underworld again. Not an evil act but emphasizes the importance of keeping your word, an important lesson in a society which relies on verbal contracts.
I can see how the two get confused. Hades is a god of death who lives in the underworld. The Christian mythology doesn’t really have a benign, neutral death figure. Hades is an easier symbol to adapt than, say, Ares, who would be my first choice of a Greek god to pit against mankind (being the god of war, strife and discontent). Most people have heard of Hades, or at least some version of him. Only those who watched the Raimi brothers’ Hercules and Xena or who study classical mythology know who Ares is.
There are times when being familiar with the source material can really suck the fun out an epic action pic. I've enjoyed much worse movies and this was no exception. But I'm not rushing out to pay money to watch the sequel.