I've generally thought that other afterlives are absolutely possible (though rarely seen) in the SPN 'verse, and those people that are bound to other gods/other religious paradigms go to whatever afterlife is overseen by those gods/religious paradigms. Hell is just where you go when the Demons (in Hell) manage to Get Your Soul through a deal, or win something in the game they play with Heaven, and maybe also with other Gods. And that the Hell/Heaven dichotomy is the one we see the most because a) it's the story Sam and Dean are involved in, and b) most of those other afterlives don't meddle so much in the modern US landscape.

So, in my mind, Ruby's backstory could totally have a non-Christian paradigm in her actual beliefs, and that's where her witchcraft knowledge/practices comes from, but some-deal-or-something-else happened during her lifetime that caused her to end up in Hell. Which is still pretty what-the-fuck, but it might give you some room to work in.

Does that make any sense?