What makes the final three and a half years so awful?
The final three and a half years are so awful because Satan, knowing his time is short, unleashes all his previously unused power upon the human race. God's wrath is then coupled with Satan's torment, intensifying the judgment during that period.
Les explains that Satan comes to Jerusalem to join the Antichrist, and he has great wrath because he knows he has but a short time—three and a half years. He will pull out all the stops, pouring out everything he can think of that he has never used before on the human race. Les uses the illustration of the Mafia threatening a businessman's family if they cannot get him to knuckle under; Satan does the same to God, tormenting the human race to get at the heart of God. God's wrath is coupled with Satan's, and that combination makes the final three and a half years what it is. Les then points to Revelation chapter 8, where the seventh seal triggers the seven trumpets.