David referred back to his actions with regards to the Amalekite who claimed that he had killed Saul (2 Samuel 1:1–16). Since he put that individual to death for a murder that took place in the context of battle, he was certain to put someone to death for a murder that had taken place whilst the victim was in the comfort of his home. Murder in battle might be understood but murder when you are lying in your bed is worse.1
11 How much more, when wicked men have killed a righteous man in his own house on his bed, shall I not now require his blood at your hand and destroy you from the earth?”