Real justice often seems in short supply.  The righteous seem to suffer while the wicked prosper (Psalm 73:2-12).  But it will not always be like that.  Injustice will stop when Jesus comes back because He will judge the world in righteousness (Acts 17:31).  The only messenger-role that the angels will have on that day is to call people away to judgement.  On Christ's orders, they will select all who have not trusted in His atoning sacrifice and throw them into the fire of God's wrath.
 
The picture is dramatic.  Jesus' hearers would be familiar with the community refuse disposal system – the undying fires which kept burning day and night.  The one outside Jerusalem was Gehenna (in the Valley of Hinnom).  Jesus said that hell would be like that, a perpetual agony accompanied by desolate remorse (Luke 16:19-31).  In that way, Christ's kingdom will be purged of rebellion and will enable those who are on the Lord's side to radiate His love and rejoice in His grace and mercy.
 
That unpopular message must be heard.  It would be the apostles' task to tell people of their peril if they reject Christ (2 Thessalonians 1:8-9; 2 Peter 2:4-9). That must be a part of the gospel (which means ‘good news’), because if there is no bad news, then the 'good news' is meaningless.  The period before Jesus returns needs the full gospel message, not just the blessings of receiving Him, but also the peril of rejecting Him.  It may not be a popular announcement, but it is better to hear and respond to the truth than to perish in denial.
 
The 'gospel of tolerance' is no gospel at all, because God will not indefinitely tolerate sin and rebellion.  Life is short: 60-90 years for most and some die much younger.  Therefore, there is an urgency to proclaim the gospel, welcoming penitents and warning rebels.  It is good that millions are listening for God to speak … but it is worse than tragic that many believers have never spoken the truth about Jesus to expose their friends and loved ones to His love or to warn them of His wrath (Matthew 3:7-10).  Every heart has a receptor for God's truth; many want to know ... but who will tell them before it is too late?