Charles Ellicott Commentary Matthew 12:34

"Ye offspring of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh." — Matthew 12:34 (ASV)

O generation of vipers — A better translation, as in Matthew 3:7, is brood of vipers. Here, the principle established in the preceding verse is now applied to explain the bitter and evil words of the Pharisees. As long as they were what they were, nothing else could be expected. Nothing but the serpent’s hiss could come from the brood of vipers; nothing but bitter words from hearts so full of bitterness.