Then answered David and said to Ahimelech the Hittite, and to Abishai the son of Zeruiah, brother to Joab, saying, Who will go down with me to Saul to the camp? Abishai said, I will go down with you.
Abishai here eagerly wanted to kill Saul, but David forbade it, because Saul was "the Lord's anointed."  This establishes the fact, as mentioned by Paul, that "The powers that be are ordained of God"  (Romans 13:1), and regardless of how wicked and tyrannical a duly-authorized head of government may be, he should not be murdered by his subjects.