Matthew 12:34 - Albert Barnes' Notes on the Bible

O generation of vipers! - Christ here applies the argument which he had suggested in the previous verse. They were a wicked race; like poisonous reptiles, with a corrupt and evil nature. They could not be expected to speak good things - that is, to speak favorably of him and his works. As the bad fruit of a tree was the proper effect of its “nature,” so were their words about him and his works the proper effect of their nature. The “abundance” or fullness of the “heart” produced the words of the lips. “Vipers” are a poisonous kind of serpents, not often a yard long, and about an inch thick, having a flat head. The males have two large teeth, through which a most deadly poison is thrown into the wound made by the bite. They are an emblem of malignity and mischief. These were strong expressions to be used by the meek and lowly Jesus; but they were not the effect of anger and malice; they were a declaration of the true character of the people with whom he was conversing - a declaration most justly deserved. See the notes at Matthew 3:7.