Matthew 23:35

“That on you may come all the righteous blood shed on the earth, from the blood of Abel the righteous to the blood of Zachariah, son of Barachiah, whom you slew between the sanctuary and the altar.”

And as a result of this behaviour they would also take the guilt of all the prophets who had died prior to this, on themselves, for all of them had died in preparing the way for the Messiah, so that to reject Him and His disciples would be to take on themselves the whole burden of guilt for those who had died before. For the idea of blood coming on someone in this way see Jeremiah 26:15. The thought has a Hebrew/Aramaic background.

Alternately the point is that God has continually held back His judgment up to this point, but now that the final day of salvation has arrived will release it on the present generation who will reject and crucify His Son. Probably there is an element of both in the words. The sins of the fathers will be visited on the children, because they are like their fathers.

For the blood of Abel the righteous see Genesis 4. He too was slain by a man who would not face up to his own sinfulness. For the blood of ‘Zachariah the son of Berechiah' we probably have to look to the Jewish tradition of the time of Jesus, which sadly is not available to us. For this was probably the Zechariah, son of Berechiah, of Zechariah 1. Certainly we know that he had many dangerous opponents whom he had outfaced (Zechariah 10:3; Zechariah 11:8), and his words had undoubtedly stirred up deep antagonism against him (Zechariah 11:8; Zechariah 11:12; Zechariah 13:7), as he described them as worthless shepherds (Matthew 11:16) so such a death is quite likely to have happened to him and to have been remembered in the tradition. He may thus well have been the last prophet to have been martyred. The description ‘between the sanctuary and the altar' is specific and suggests some specific and well known tradition. This makes it unlikely that this refers to Zechariah the ‘son' (probably grandson, and therefore he could have been a son of Berechiah, which was not an uncommon name, compare 1 Chronicles 6:39) of Jehoiada, who while he was slain in the courtyard of the Lord's house (2 Chronicles 24:21), was not said to have been slain in this specific place (the priestly section of the courtyard). If Jesus had been referring to him why would He not have cited what Scripture actually said about him? Other suggestions include the obvious one that it was an unknown prophet of whom we know nothing. But he was clearly well known in Jesus' day.