This is a metaphor. Here "offspring" means "having the characteristic of." Vipers are a kind of dangerous snakes and represent evil. This can be stated as a separate sentence. Alternate translation: "You evil poisonous snakes! Who" or "You are evil like poisonous snakes! Who" (See: [[rc://en/ta/man/translate/figs-metaphor]])
John uses a question to rebuke the Pharisees and Sadducees because they were asking him to baptize them so that God would not punish them, but they did not want to stop sinning. 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." (See: [[rc://en/ta/man/translate/figs-rquestion]])
The word "wrath" is being used 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" (See: [[rc://en/ta/man/translate/figs-metonymy]])