It is the bloody business which informs
Thus to mine eyes.
This is Shakespeare’s first explicit reference to the illusion of the dagger, and subsequent illusions of Banquo for Macbeth and the “damned spot” for his wife, being caused by guilt. He is saying that the “bloody business” or murder is causing him to see the dagger. It shows that Macbeth is extremely hesitant to commit this murder, and that his morality is rebelling against it, even to the point of his senses lying to him.