Adam Clarke on Luke 23:31

If they do these things in a green tree - This seems to be a proverbial expression, the sense of which is: If they spare not a tree which, by the beauty of its foliage, abundance and excellence of its fruits, deserves to be preserved, then the tree which is dry and withered will surely be cut down. If an innocent man be put to death in the very face of justice, in opposition to all its dictates and decisions, by a people who profess to be governed and directed by Divine laws, what desolation, injustice, and oppression may not be expected, when anarchy and confusion sit in the place where judgment and justice formerly presided? Our Lord alludes prophetically to those tribulations which fell upon the Jewish people about forty years after. See the notes on Matthew 24:1-51 (note).