What does Matthew 21:39 mean?

This concludes this parable about wicked tenants and the owner of the vineyard they are responsible to tend. Instead of paying their rent to the owner, in the form of a share of the crops, these tenants have mistreated and killed every servant the owner has sent to collect from them (Matthew 21:33–37). Now the owner has sent his son (Matthew 21:38), and the tenants have thrown him out of the vineyard and killed him, as well. They imagine that with the son dead, they will be able to take his inheritance.

Perhaps Jesus' listeners in the temple do not see it, yet, but His story is meant to show how Israel's religious leaders—including those Jesus is talking to—have failed to tend the vineyard of Israel faithfully. The have mistreated God's prophets, servants sent to collect righteousness from Israel (Acts 7:52), and they will soon kill God's Son (Matthew 17:22–23). Jesus is once again predicting His own death, this time in parable form and to the very men who will condemn Him in the coming days.

The question Christ poses in the following verse will begin to make this plain.