Albert Barnes Commentary Matthew 21:38

"But the husbandmen, when they saw the son, said among themselves, This is the heir; come, let us kill him, and take his inheritance." — Matthew 21:38 (ASV)

But when the husbandmen, etc. They determined to kill him; and as he was the only son, they supposed they could easily seize on the property. It was rented to them, was in their possession, and they resolved to keep it. This circumstance has probably no reference to any particular conduct of the Jews, but is thrown in to keep up the story, and fill up the narrative. An heir is one who succeeds to an estate, commonly a son; an inheritance is what an heir receives.