Do you mean why did Macbeth forget to *leave* the daggers? (If so, that's an excellent question). Macbeth does not tell us directly, but we know he has been extremely ambivalent about killing Duncan. We also know that he is already exhibiting signs of guilt (he thinks he hears a voice saying Macbeth does murder sleep), and that he is not thinking clearly. He has fought numerous battles, so he should not be afraid of blood; neither should his mind wander after a killing, yet it does. He is not one to make mistakes in battle, so this mistake is all the more poignant. Perhaps the daggers return with him because he cannot leave behind him what he has done.
After the killing of King Duncan and drugging the guards into a deep sleep, the "knife" purpose was to implicate the guards and pushed all the blame of the murder of the king towards them. It was supposed to be left behind by the sides of the guards.
His wife told him that he had forgotten to leave behind the knife in the scene of the grisly murder, but he feared to see the victim after the aftermath of his grisly murder. I think he was guilty conscience of the horrible thing he had done to harm an innocent life, undergoing terrible pangs of self-doubt and self-awareness. T
his shows us that despite being a cold-blooded senseless murderer, he still have some "human" feelings and emotions, showing that he is not all evil inside