What does Mark 12:3 mean?

In the parable of the tenants, the landowner sends his servants to the vineyard to gather the fruit the tenants owe him. In Jewish history, God sent prophets to the people to gather what the Jews owed God. In return for the blessings God gave the Israelites, He expected worship, sacrifice, obedience, and devotion.

The Jews' treatment of the prophets showed directly how the people felt about God. The prophets came in God's name, with His commission and His authority. In response, the prophets were beaten (Jeremiah 20:2–4), thrown into a cistern (Jeremiah 38), and killed (1 Kings 18:4; 2 Chronicles 24:20–22; Jeremiah 26:20–23; Hebrews 11:32–38). The last Old Testament-style prophet is John the Baptist. After a hostile relationship with the Jewish leaders and a public fight with the Tetrarch, John was killed for doing his job: telling the truth and calling people to do what they know is right (Mark 6:14–29).

The prophets' job was merely to call the people to give God His due. This parallels how the messenger of this parable has come to gather some of the fruit of the vineyard for the owner. A vineyard takes four years to grow, and the owner has been away since he planted the vines. The tenants may think that because the owner is far away, he won't retaliate for the mistreatment of his servant. Eventually they believe that if they kill all the landowner's representatives, they can lay claim to the vineyard as legal squatters.

Seeing the parallel to the Old Testament, it's understandable why removal from the land—exile—was a consequence of Israel's disobedience.