Albert Barnes on Matthew 23:31

Ye be witnesses unto yourselves. The emphasis, here, lies in the words "unto yourselves." It was an appeal to their conscience. It was not by their building the tombs that they were witnesses that they were the children of those who slew the prophets; but in spite of all this pretence to piety--under cloak of all this profession--they knew in their consciences, and were witnesses to themselves, that it was mere hypocrisy, and that they really approved the conduct of those who slew the prophets.

Children of them, etc. Resembling them; approving their conduct; inheriting their feelings. They not only showed that they were descended from them, but that they possessed their spirit, and in similar circumstances would have done as they did.

(n) "which killed" Acts 7:52, 1Thes 2:15