By this point, Macbeth is unable to be rational in his judgments or to be able to tell good from evil. He sees Macduff as a threat to his future, so he feels taking quick action against him and his family will insure that any blood line that might threaten his future will be stopped. He doesn't stop to think about what he is doing. He murders in order to stay powerful and in control. The senseless murders of Lady Macduff and her son show how much Macbeth has degenerated into a cold-blooded killer. These killings reinforce Macbeth's character flaws which lead to his tragic end.