The people to whom Jesus is speaking will be held responsible for the murder of the prophets by their ancestors. Alternate translation: "Therefore, God will hold this generation responsible for all the deaths of the prophets that people have killed" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-activepassive]])
The "blood ... shed" refers to the blood spilled when they were killed. Alternate translation: "the murder of the prophets" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The author speaks of the world as if it were a building set on a foundation. Alternate translation: "since the creation of the world" or "since God created the world" (See: [[rc://en/ta/man/translate/figs-metaphor]])