Yes, Teribus, I've been pondering Jack's comment over coffee, and I'm coming to the same conclusion. The first line is "now"; from line 2 to the end of v.4 is a flashback all about Mary and Bothwell; and v.5 brings us back to "now". Makes sense now, thanks everyone.

Take what Teribus says about the lawlessness of the place, and add to that the association with the 13th century warlock De Soulis, you've got a really scary setting for a song...