In the week before Jesus was crucified, even though He knew exactly what would happen (Luke 9:22) He was not afraid of those plotting to kill Him.  He chose to teach publicly in the temple (Luke 20:1) so that the disciples could later understand how to teach essential doctrines to the Early Church (John 12:16).  This parable was a prophetic description of the hatred of the religious establishment against Him.  The priests and religious lawyers who were hovering in the back of the crowd knew that the story described them, and it intensified their lethal hatred (Luke 20:19).  

But they had not been the first to oppose God's right to rule.  The Old Testament is full of self-justifying leaders who refused to fully obey God's Word.  And when He sent prophets to tell them the truth, the messengers were ignored, persecuted or killed.  This parable starts with their story.  Paradoxically Israel in Jesus’ day revered the prophets who their earlier countryfolk vilified (Matthew 23:29-31).  But the prophets spoke about God’s people as His vineyard, created and owned by God.  The well-known words of Isaiah 5:1-7 would have come to their minds as Jesus told this parable.

The farmers were the family, tribal and national leaders of Old Testament Israel who were trustees of the 'testament' (covenant).  They were God's stewards to look after His people and land.  However, despite having God's written instructions, He often seemed to be absent (although He was not), and so the leaders developed their own agenda.  They turned away from God's law, indulging their own fantasies.  Alas, this included the kings as well - who were intended to be the "pastors" of God's people.  So God sent prophets to remind them of the covenant and rebuke their rebellion (Ezekiel 34:1-24).  However, God's prophets were often treated badly, even though their warning message was also a gracious call to repentance and reconciliation (Hebrews 11:33-38).
 
God is not physically visible today but He is still watching over His world of which He is the owner.  He has every right to call people to order and expect obedience, service and worship.  In the New Testament church, the gospel contains the new covenant, and the church is commissioned to proclaim the truth about God's Kingdom - called to announce Jesus as the only Saviour and command repentance in His Name (Luke 24:47).  However, the response of many is to ignore the gospel and increasingly to persecute those who speak the truth about Jesus.  But God is still the ruler of His Kingdom and the owner of this world.  He has the authority to say what is right and the power to enforce it.  The privilege of believers in Jesus is to announce forgiveness of sins and freedom from condemnation for all who repent and receive Jesus Christ.  If you are in His family, it is not all about you, but ensuring that God is glorified.