Generally they weren’t depicted as ‘evil’ so much as necessary components of the divine ecosystem.
The idea of ‘evil’ as we know it largely developed out of monotheistic ideals and the idea that there was a perfect single good and that any opposition to that was inherently evil.
Zeus wasn’t associated with the underworld, but he was a dick and not always good. And Hades wasn’t always bad. In polytheism the gods were often a projection of spectrum of human qualities and behaviors and not monolithic.