Albert Barnes on Matthew 23:33

Ye serpents. This name is given to them on account of their pretending to be pious, and very much devoted to God, but being secretly evil. At the heart, with all their pretensions, they were filled with evil designs, as the serpent was, Gen 3:1-5.

Generation of vipers. Mt 12:34.

Damnation of hell. This refers, beyond all question, to future punishment. So great was their wickedness and hypocrisy, that if they persevered in this course, it was impossible to escape the damnation that should come on the guilty. This is the sternest language that Jesus ever used to wicked men. But it by no means authorizes ministers to use such language to sinners now. Christ knew that this was true of them. He had an authority which none now have. It is not the province of ministers to denounce judgment, or to use severe names; least of all to do it on pretence of imitating Christ. He knew the hearts of men; we know them not. He had authority to declare certainly that those whom he addressed would be lost; we have no such authority. He addressed persons; we address characters.

(p) "generation of vipers" Mt 3:7