It depends ultimately on the context and the rhetorical intentions of the author/publication.
Capitalization of "Hell" reinforces notions, attitudes and opinions of Hell as the proper name for a real and specific place. Non-capitalization of "hell" reinforces concepts of hell as a common noun that encompass ideas of hell as a state of being as opposed to a specific place, a generic term for the pan-cultural locations (and pseudo locations) such as Gehenna, Pandemonium, Tophet, etc., (which are regularly capitalized), and "hell" as an abstract notion that does not exist in a literal form.
Merriam-Webster's Dictionary uses only the uncapitalized form and make no mention of "hell" or "heaven" as being capitalized.
The American Heritage Dictionary uses the uncapitalized forms of "heaven" and "hell" as the entry words, but notes at the head of relevant entries "Often Hell" or "Often Heaven."
For secular publications, it is clear that the uncapitalized form is preferred. For religious publications, where hell is more likely to be reified in literal ways and wherein that concept is part and parcel of the rhetorical purpose of the text, capitalization of Hell and Heaven becomes more common and accepted, and perhaps even preferred.