The narrator describes the old man's eye as "the eye of a vulture": it is light blue and seems to have a kind of film over it. It seems possible, then, given his age, that the old man has cataracts, as they would give it this appearance and prevent him from seeing out of this eye. The narrator says, "Whenever it fell upon me, my blood ran cold [...]." By describing the man's eye as being like a vulture's eye -- an animal associated with death and decay -- Poe demonstrates how deeply the narrator hates the old man's eye and what it symbolizes (i.e. mortality and the fact that we will all die and decay someday). Further, the fact that the narrator describes the eye as making his blood go cold shows us just how much he dreads and fears it. He does not seem to hate the old man himself, as he says,

I loved the old man. He had never wronged me. He had never given me insults. For his gold I had no desire.

Therefore, we see that the narrator does not hate the old man himself but, rather, what the old man's eye represents. The intensity of his hatred is further revealed by his calling it the man's "Evil Eye." On the eighth night of stalking the old man, the narrator says that he "grew furious" as he looked upon the eye, and he describes it as "hideous," so much so that it "chilled the very marrow of [his] bones." Such language continues to convey the narrator's visceral loathing of the old man's eye.