You took Saul's kingdom, and now the Lord is punishing you for murdering so many of Saul's family. The Lord has given the kingdom to your son Absalom, and you are ruined, you murderer!”
If you did this, you would defile the land where you are living. Murder defiles the land, and except by the death of the murderer there is no way to perform the ritual of purification for the land where someone has been murdered.