The old man is the antagonist in the story, not because he is bad, but because he is working against the narrator in some way. There is not enough information given about the old man to cast a specific person to play him. Anyone who as ever been judged for their physical appearance can qualify for this role because the narrator had nothing against the old man except solely for his hideous eye. However, for the physical aspect of the old man, I would cast Michael Gambon because he is how most old men are ideally pictured.