Well, he actually did do a couple of horror films late in his career, around 1972: NECROMANCY, which as I said is pretty bad, and the Belgian film MALPERTUIS, which I think is pretty good, although Welles is only in it for 10 minutes or so. The pop-Shakespearean elegance of the Hammer films, however, is something I think Welles would've fit quite nicely in too, and it's too bad he didn't do at least one, or at least an AIP picture with Vincent Price. Of course, Welles' film versions of MACBETH and OTHELLO are both often described as having some qualities of the classic horror films of the 30's. I know Jonathon Rosenbaum compared OTHELLO to FW Murnau's NOSFARATU.
BTW, MALPERTUIS is now available on a European DVD, if anyone has any further info on it. Also, on DVD now in Europe is Welles' only appearance in a western, TEPEPA (which is also a pretty good movie). Has anyone seen either of these?