# You offspring of vipers, who

Here "offspring" means "those who are like something." Vipers are dangerous snakes and represent evil. Alternate translation: "You evil poisonous snakes! Who" or "You are evil like poisonous snakes! Who" 

# who warned you to flee from the wrath that is coming?

John rebukes the Pharisees and Sadducees. Alternate translation: "you cannot flee from God's wrath like this." or "do not think that you can flee from God's wrath just because I baptize you."

# flee from the wrath that is coming

The word "wrath" is used here to refer to God's punishment because his wrath precedes it. Alternate translation: "run away from the punishment that is coming" or "escape because God is about to punish you"