Concise Bible Dictionary:

All we learn from the passages that speak of the viper is that its bite was poisonous: “the viper’s tongue shall slay him.” When one fixed on Paul’s hand they expected that he would drop down dead. What species of serpent is alluded to is unknown. It is only otherwise referred to in the New Testament as symbolical of evil ones. John the Baptist called the multitude who came to be baptized “a brood of vipers,” and the Lord applies the same term to the scribes and Pharisees, showing the deadly character of their opposition (Job 20:1616He shall suck the poison of asps: the viper's tongue shall slay him. (Job 20:16); Isa. 30:66The burden of the beasts of the south: into the land of trouble and anguish, from whence come the young and old lion, the viper and fiery flying serpent, they will carry their riches upon the shoulders of young asses, and their treasures upon the bunches of camels, to a people that shall not profit them. (Isaiah 30:6); Isa. 59:55They hatch cockatrice' eggs, and weave the spider's web: he that eateth of their eggs dieth, and that which is crushed breaketh out into a viper. (Isaiah 59:5); Matt. 3:77But when he saw many of the Pharisees and Sadducees come to his baptism, he said unto them, O generation of vipers, who hath warned you to flee from the wrath to come? (Matthew 3:7); Matt. 12:3434O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh. (Matthew 12:34); Matt. 23:3333Ye serpents, ye generation of vipers, how can ye escape the damnation of hell? (Matthew 23:33); Luke 3:77Then said he to the multitude that came forth to be baptized of him, O generation of vipers, who hath warned you to flee from the wrath to come? (Luke 3:7); Acts 28:33And when Paul had gathered a bundle of sticks, and laid them on the fire, there came a viper out of the heat, and fastened on his hand. (Acts 28:3)).