Ye serpents, ye offspring of vipers (opheis gennēmata echidnōn). These blistering words come as a climax and remind one of the Baptist (Mat 3:17) and of the time when the Pharisees accused Jesus of being in league with Beelzebub (Mat 12:24). They cut to the bone like whip-cords.
How shall ye escape (pōs phugēte). Deliberate subjunctive. There is a curse in the Talmud somewhat like this: “Woe to the house of Annas! Woe to their serpent-like hissings.”