William Burkitt on Matthew 12:34

Note here, 1. The fervency and zeal of our Saviour's spirit in the compellation given to the Pharisees: he calls them a generation of vipers: intimating that they were a venomous and dangerous sort of men.

Learn hence, That it is not always railing and indiscreet zeal to call wicked men by such names as their sin deserves.

Observe farther, From our Saviour's saying, that out of the abundance of the heart the mouth speaketh; that the heart is the fountain both of words and actions: according as the heart is, so is the current of men's words and actions, either good or evil.