The mission of John sounds much like some of the old "fire and brimstone" preachers I heard about when I was very young. It seemed like they were determined to "scare the devil" out of the people. That style of preaching is not heard so much any more, but it is no less valid.
We have been fed the pap that God is all love. He is. But He is also a righteous God and will not stand for unrighteousness in His presence.
John's "brood of vipers" comment is directed to the crowds in this passage, but in Matthew, it is directed to the Pharisees and Saducees. In our day, these would be the church-goers and those who are elders or deacons or generally active in the works of the church.
Our salvation is not dependent upon our position in the church. It depends on our relationship to Christ. The only way we can stand in the presence of a Holy God is cloaked in the righteousness of His Son, cleansed by His blood, dependent upon Him for every breath.
Our good works - or fruit of repentance as John puts it - is not our salvation. It is evidence of our commitment to Christ. If you belong to Christ, your life will show it. If you spend all your time outdoors, you skin will show the evidence of exposure to sun and wind. If you spend time in God's presence, your life will reflect Him.
Prepare the way. Examine yourself for fault and sin. Ask the Lord to show you your hidden self. We all have corners in our lives that are forgotten and cobwebby. Seek His light in your life and He will show you those hidden corners and help you to clean them up.
We're preparing the way of the Lord - at least in our own lives.
Shine the light of Your love into the dark corners of our hearts. Help us to see what we need to dust off and use again, as well as what we need to get rid of entirely. Grant that we may be prepared in heart and mind for doing Your bidding at all times.
In Jesus' most precious name.