What does Matthew 21:38 mean?
Jesus, in the temple, is telling a parable meant to illustrate the relationship between God the Father and Israel's religious leaders. The nation's leaders are represented by the tenants: renters of a vineyard. God owns the vineyard, which is Israel. He has repeatedly sent servants to collect the rent from the tenants, but they have mistreated and even killed the owner's servants. The servants may be understood as the prophets of old (Acts 7:52; Matthew 23:37).Now the owner of the vineyard has sent his own son, thinking the tenants will respect him. This is a gracious step, considering that the owner could have responded with violence or revenge of his own. Instead of responding in obedience, the tenants see an opportunity to have the son's inheritance for themselves by killing him. They hope to own the vineyard by eliminating the future rightful heir.
It's important when hearing any parable not to over-analyze the details. Each parable is meant to make a specific point, not to make every detail line up exactly with a precise person or action in the real world. The leaders of Israel were not literally attempting to become God, or to steal the inheritance of His Son. Broadly, Jesus is describing what Israel's leaders have done to God's servants over time and what they are about to do to Himself, God's Son: to reject Him with violence and sin.