I’m going to disagree about the lighting, sort of, though it all depends on what you are trying to portray 
I think creepy is the way to go, but I wonder where that creepiness is coming from?
If it is emanating from the man himself, then go with darker shadows to accentuate him.
Personally, I’d prefer it to come from the situation, the exploitation involved in a freakshow. To emphasize this you may want to add more light, especially to the figure. This is like the difference in lighting between eroticism (dark, soft and sultry) and pornography (bright, harsh and revealing). He’s obviously not going anywhere, so maybe one way of doing this would be to enlarge the window at the back so he is rimmed in stark white light.
To this end I’d also give the man a blank, unengaged stare at his lap (as a truly brainless man would do) and reduce him completely to just an object.
I don’t really understand the crucifix. To my mind a brainless man would have no religion, so someone else must have put it there as a talisman and act of kindness. If you went with the exploitation line, you could maybe swap it with something else that was put there as an act of cruelty to mock.
Having said that, I certainly don’t want to hijack your picture, so is there a specific message or story you would like to portray?