Why does Jesus admonish the scribes and Pharisees in Matt. 23:29?
We need to read the whole thing in contexts:
Matthew 23:27-28 is that the Pharisaic oral tradition, "the death of the righteous can atone for the sins of that generation" has been used to deem the prophets as sin sacrifices...
In Mark 7:6-13 Yeshua states they've done away with the Torah because of these oral traditions.
Thus they are like white washed buildings that appear clean, yet it doesn't clean the inside, and instead defiles it, by honouring murder as atonement.
Matthew 23:29-32 states that those who condone the murdering of God's messengers, and endorse it by their declarations, prove they'd murder the prophets as well.
Matthew 23:33-38 shows that due to Judah murdering prophets, and deeming them as an atonement, they shall be cut off; then all the world will be brought into condemnation, when they teach the Gentiles to do the same (Christianity is Pharisaic - John, Paul, Simon the stone (petros)), then Judgement will come on the whole world (wicked generation).
In my opinion.