His winnowing fork is in his hand, and he will clear his threshing floor and gather his wheat into the granary, but the chaff he will burn with unquenchable fire. Kingdom here indicates an active ruling, not a static place or group of people. People believed that a prophet like Elijah would come just before the end of time. Pharisees and Sadducees were highly respected in the Church.
Vipers are extremely poisonous snakes, which look like a good dry branch.
St. Paul was bitten by a viper once when he was gathering wood. Therefore John dipped people in water, but Jesus dips people in fire and wind. This dipping can be violent, can kill, and relates to the image of death and resurrection. It does not mean having an emotional binge of remorse over a past that is now beyond our control.