John Gill Commentary Matthew 23:31

"Wherefore ye witness to yourselves, that ye are sons of them that slew the prophets." — Matthew 23:31 (ASV)

Therefore you are witnesses to yourselves
Or "against yourselves", as the Syriac reads; for what they said was a plain acknowledgment, and a full confession, what their fathers had done, and whose offspring they were; and from whom better things were not to be expected; since they were their fathers' own children, and of the same temper and disposition with them:

that you are the children of them that killed the prophets .
They plainly owned, that their fathers killed the prophets, and that they descended from them; though they meant not so much to reproach, their ancestors, as to give themselves a greater character; yet it did not with those, that knew them; not with our Lord: for as their own words testified against them, that they were a seed of evildoers; their practices showed them to be of the same spirit and principles with their progenitors.