"Blood will have blood"
In act three scene four they mention a quote saying "blood will have blood". Macbeth is a murderer who's crimes have not yet noticed. Although he has committed these crimes and continues to do so he knows that he will not get away with it for long. When Macbeth goes to say "blood will have blood" he is trying to say that the blood of the victim will soon find the blood of the murderer, and a murder will always be discovered.