1st, 2nd, 3rd stanzas - A dialogue between demonic entities, apparently vampiric in nature and possibly acting as psychopomps. However, Roman centurions might just have well have sung the 3rd stanza, and much of these three stanzas' content might be ascribed to the speaker in the 4th 5th and 6th as well.
4th, 5th, 6th stanzas - Possibly a speech delivered by Satan to his fallen compatriots in Hell, as depicted in Milton's Paradise Lost (fourth section of lyrics).