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.
Macbeth had resolved himself into a much more stereotypical villain and his ambition of controlling the whole empire turn himself into a cold-blooded monster and downgrading himself into a lowly coward. Macduff had some suspicions and queries over the death of King Duncan and think that something is lurking beneath the waters, so Macbeth thought that he would be a obstacle and stepping stone towards his success, he decided to take quick action against his power by killing anybody who stand before his power, by severing bloodlines and killing innocent families to demonstrate his power and stay in control in the empire, so his senseless killing of Macduff's family proved to us how terrible and horrifying his true character is.