The Bible contains songs about salvation and songs about judgement (Rev. 15:3-4; 19:1-3). Not of course that they are held far apart, or that you can sing the one and go mute on the other because you only want to sing the nice ones. You can't skip verses about judgement when you sing Exodus 15:1-18.
God's acts of retributive justice are praiseworthy because he is just (Rev. 16:5-6), and because he is just in his holy character we can say 'true and just are your judgements' (Rev. 16:7) and worship him because his 'righteous acts have been revealed' (Rev. 25:4)..
Where did we learn to sing songs about judgement? Surely it is because we have joined the choir that sings a 'new song' about the Lamb slain, who bore the wrath in our place and purchased men for God (Rev. 5:9-10). If you don't want to sing God's praises for his true and just judgements you will also find yourself departing from the original lyrics of that 'new song'.
Our trouble, if we refuse to sing these songs, is that we are like the failed applicants at the audition stage on the X Factor or American Idol. We think we know enough about sin, and the judgement that sinners deserve, that we don't need to be corrected when we are off-key. We think we are pitch perfect already.
Listening to these discordant notes is not an expert behind a desk, but the Composer, Lyricist and Conductor of heaven's songs and heaven's choirs. And he is seated upon a throne.
When it comes to singing and speaking about judgement don't think that you have the right to judge the Judge when you are hitting all the wrong notes.
Today is the day of salvation. Earnestly, winsomely, prayerfully, we plead with people to face up to the reality of judgement and to come to Christ. Their lost condition grieves us and moves us. We long to have them join us in singing the song of the Lamb. But, this day of opportunity will end, and our God will not be unjust when Jesus judges the living and the dead..
The final judgement is not a final act of cruelty but an awesome, public, display of justice. Or, in other words, sinners will be treated as their sins deserve -- 'they were judged, each one of them, according to what they had done' (Rev. 20:12, 13; 'He will render to each one according to his works' Rom. 2:6; see also Psalm 62:12; Prov. 24:12; Job 34:11; Jer. 17:10; 32:19; Matthew 16:27).
Here is Augustine's take on the matter:
Now the reason why eternal punishment appears harsh and unjust to human sensibilities, is that in this feeble condition of those sensibilities under their condition of mortality man lacks the sensibility of the highest and purest wisdom, the sense which should enable him to feel the gravity of the wickedness in the first act of disobedience.
Luther was singing from the same hymn sheet:
Since God is a just Judge, we must love and laud his justice and thus rejoice in God even when he miserably destroys the wicked in body and soul; for in all this his high and inexpressible justice shines forth. And so even Hell, no less than Heaven, is full of God and the highest good. For the justice of God is God himself; and God is the highest good. Therefore even as his mercy, so his justice or judgement, must be loved, praised, and glorified above all things.