John Chrysostom

Not because they build, nor because they blame the others, doth He say, woe, but because, while both thus, and by what they say, they are pretending to condemn their fathers, they do worse. For in proof that the condemnation was a pretense, Luke saith, ye do allow because ye build; for, "Woe unto you," saith He, "for ye build the sepulchers of the prophets, and your fathers killed them. …

Homily on the Gospel of Matthew 74 ↗

Theophylact of Ohrid

He deplores them, not for building tombs for the prophets, for that is pleasing to God, but for doing these things in pretence, and for passing judgement on their fathers while doing worse things themselves, and exceeding them by far in malice. They lie brazenly when they say, "If we had been in the days of our fathers, we would not have killed the prophets," for they were rabid with desire to kill the Master of the prophets. Therefore Christ says:

Commentary on Matthew ↗

Jerome

Woe to you, scribes and Pharisees, hypocrites! For you build the tombs of the prophets and decorate the monuments of the righteous, and you say, 'If we had lived in the days of our fathers, we would not have been partners with them in shedding the blood of the prophets.' Thus you bear witness against yourselves that you are sons of those who murdered the prophets. …

Commentary on Matthew ↗

Thomas Aquinas

Then when he says, woe to you, (...) who build the sepulchres of the prophets, he rebukes them for the pretense of piety, and he does two things. First, he sets forth their pretense; secondly, their cruelty, at wherefore you are witnesses against yourselves, etc. Likewise, they pretend in two ways, by deeds and by words. Hence he first rebukes them regarding their deeds; secondly, regarding their words. The second is at and you say: if we had been, etc. He says, then, woe to you who build the sepulchres of the prophets. …

Commentary on Matthew ↗