I have gone out, a possessed witch, haunting the black air, braver at night; (1-2)
Why might our speaker be braver at night? We get the sense that she enjoys being outside of the hustle and bustle of everyday life. It's when she's alone that she can allow her real identity to come out.
dreaming evil, I have done my hitch over the plain houses, (3-4)
Notice how there don't seem to be any fellow witches in this picture? Our speaker's out there on her own, above all the houses where everyone else is doing their living. Describing the houses as "plain" suggests that they're both normal and, well, boring.
lonely thing, twelve-fingered, out of mind. (5)
Yup, lonely and isolated are pretty much the same thing. You don't get much more explicit than this, folks!