John Gill Commentary Luke 11:51

"from the blood of Abel unto the blood of Zachariah, who perished between the altar and the sanctuary: yea, I say unto you, it shall be required of this generation." — Luke 11:51 (ASV)

From the blood of Abel
"Righteous Abel", as in (Matthew 23:35) and so read the Arabic version here, and two manuscripts in the Bodleian library, and three of Stephens's copies; the Persic version renders it, "innocent Abel": he is mentioned because he was the first man that was slain, and he was slain because of his righteousness.

Unto the blood of Zacharias ;
in the Cambridge copy of Beza's, it is added, "the son of Barachias", as in (Matthew 23:35) and so the Arabic version, "the son of Barasciah"; who he was, (See Gill on Matthew 23:35)

which perished between the altar and the temple ;
or "the house", that is, the holy place: and the Ethiopic version renders it, "the holy house"; here he died, being slain by the Jews; see the note, as before.

Verily I say unto you, it shall be required of this generation ;
as it was at the time of the destruction of Jerusalem.