Jesus is teaching about the character of God.  God created Isreal and set standards for His people to live righteously.  The religious leaders to take care of Isreal and communicate God's desires to the people - by encouraging people to live correctly and setting an example.  But whenever He sent prophets to Isreal to communicate His desires for Isreal (like to stop sinning and to care for the poor and vulnerable members of society).  These prophets were killed, mistreated and rejected.  Eventually, God sent His Son.  Our God is a persevering God.

Jesus is also teaching about the character of the religious leaders.  The tenants in the parable weren't concerned about respecting the landowner or about giving him the fruits of his land.  The religious leaders of the Jews were corrupt and not respecting God or giving God the fruits of Isreal.  They were more concerned with using the system to get money for themselves than about using money to further God's work.

Jesus is also foretelling His death.  The tenants in the parable kill the landowner's son.  Jesus will also be murdered.