“Thus you witness to yourselves, that you are sons of those who slew the prophets. Fill you up then the measure of your fathers.”
Jesus then points out to them that by all this they are simply drawing attention to the fact that they are the sons of those who slew the prophets. They are of the same blood and, although they may not think it, are demonstrating the same attitude, for they are at this very time plotting His death.
‘Fill you up then the measure of your fathers.' This was a sarcastic way of telling them to carry on their plots against Him. It was all that could be expected for they were like their fathers and could therefore only be expected to behave like them. ‘The measure' probably indicates that they will finally fill up what their fathers have commenced, referring to the limit put by God on the amount of sin He will tolerate, which once it is reached causes Him to act (compare Genesis 15:16; 1 Thessalonians 2:16).