Matthew 23:34

The antecedent of "therefore" (Gr. dia touto) is the Jews’ execution of the prophets that God had sent them in the past (Matthew 23:29-30; cf. Matthew 22:3-10). Because the Jews had rejected the former prophets Jesus would send them additional prophets, wise men, and teachers. These the Jews would also reject, filling up the measure of their guilt to the full. This is probably a reference to the witnesses that followed Jesus and appealed to the Jews to believe in Him (Acts 3:19-21; Acts 7:2-53; cf. Matthew 5:10-12; Matthew 9:37-38; Matthew 28:18-20).

Jesus would not establish His kingdom then because Israel rejected Him as her Messiah. However, now Jesus revealed that God would punish the generation of Israelites that rejected Him and the apostles who would follow Him in an additional way. This included the destruction of Jerusalem and the dispersion of the Jews from the Promised Land. Jesus clarified these events in the Olivet Discourse that follows (chs. 24-25).

Since the Jews did not have the authority to crucify people, we should probably understand Jesus’ reference to them crucifying some of these witnesses in a causative sense. They would cause others, notably the Romans, to crucify them (cf. Matthew 10:24-25).