Woe to you who whitewash: According to the law, contact with a dead body, even if one was unaware of it, caused ritual impurity (cf Nm 19:11-22). In order to protect themselves from this possibility, Jews whitewashed the tombs.
Matthew compares the outside of the tomb to the appearance of the scribes and Pharisees; both nice and clean. However, the inside hides decaying flesh, stench, and filth. Likewise, inside the Pharisees there is hypocrisy, the essence of sin, and evildoing, lawlessness and impurity. And so, Matthew warns the members of his church to be what they appear to be. Otherwise, they are mere scribes and Pharisees in disguise.
The seventh and final woe denounces the scribes and Pharisees for being just like their ancestors, who murdered the prophets and the righteous. The scribes and Pharisees have restored the tombs of the prophets and adorned the monuments of the righteous and by doing so attempt to disown the murders done by their ancestors.
However, Jesus declares that they are the heirs of murderers. Reflecting a Jewish notion that there is an allotted measure of suffering that has to be completed before God’s final judgment takes place, Jesus orders the scribes and Pharisees to fill up what their ancestors measured out.
For Matthew, this points towards the rejection, suffering, and death both of Jesus and of the church. In Matthew’s view, those who build memorials and disdain participation in the suffering and death of the righteous are the real murderers. Outer behavior does not necessarily reveal authentic inner disposition.