In fairness, I didn't think all that much of Demon. It was essentially what I'd catalogue as a 'nice' picture, nothing really standing out all that memorable in the performances or the practical effects (which were few and far between). Robert Clarke jumped at the chance to dabble once more in Science Fiction and Fantasy as one of his recent outings had actually done quite well at the box office. As you can see by the poster above, he not only starred in this but also produced and directed. I believe I'd also read he didn't exactly contribute to the script so much as he came up with the core idea and then let the screenwriters have at it. So, yes, I think this one also qualifies as a 'vanity project,' though some might disagree.
Furthermore, I distinctly recall the flick being reasonably light on time spent with the creature itself. In any Jekyll & Hyde story (of which this qualifies), I'd argue that the split should be, at least, 60/40. Naturally, the sane half of the split personality would be provided more screen time -- the story needs the set-up, and the inter-relationships between all of the players would also require some exploration -- but I think this one had very, very little time actually in costume. In fact, I'm pretty sure the bulk of it was in the finale; and -- at that point -- monsters usually are showing up to deliver a swan song, aren't they? Sigh. It ain't easy being green.
Like many who write and think about film, I have the compunction to care more about the creature than I do the -- ahem -- mere mortals. It's the monster that I typically experience more empathy for: he didn't ask to be created, nor did he ask for whatever hungers drive him to commit such nefarious and sometimes bloody deeds. While Clarke's picture had its heart in the right place, what was truly needed was a thicker wallet.