What does Luke 20:12 mean?

The third servant is sent but his fate will be no better than that of the first two. Jesus is telling the parable of the wicked tenants (Luke 20:9–18). A landowner has planted a vineyard. When the vines are ready, he sends servants to his tenants to collect some of the grapes. Instead, the tenants beat the servants and give them nothing.

In the parable, the owner is God, the tenants are leaders, the vineyard is God's people, and the servants are prophets. God charged the leaders to guide the people rightly. They were to teach a proper relationship with the Lord, and in return they would receive the Lord's blessings. To ensure they were rightly leading the people, God occasionally sent prophets. But Israel's leadership does not show a history of treating prophets well. There are a few exceptions, such as Isaiah with king Hezekiah, but Isaiah was probably murdered by Manasseh, Hezekiah's evil son.

Jesus drives the point home, here, by mentioning three servants. All of them suffer the same fate. In Matthew's account, some of the servants are killed (Matthew 21:35). Finally, the landowner sends his son. The tenants kill him, too.

At that point, the landowner is finished. He drives the tenants out, kills them, and finds replacements. The Jewish leaders who are listening know Jesus is speaking about them: He's saying God is going to remove their authority as leaders of His followers and replace them. That just makes them want to destroy Jesus more (Luke 20:19).