The idea that God is a righteous judge offends many modern people. It conjures up image of a wrathful God who is distant from us. The character Aslan in the Chronicles of Narnia offers a refreshing read on the nature of God. When Lucy asked if Aslan is safe, she was told that "of course he is not safe, but he is good". God's judgment needs to be understood in conjunction with God's goodness and kindness. Psalm 136 helps us connect the dots by doing precisely that. "God struck down the firstborn of Egypt, His love endures forever." A few verses later it says, "God swept Pharaoh and his army into the Red Sea, His love endures forever." Those statements are self-contradictory if they are read out of context. But if we understand them in the context of God's faithful, covenantal love for his people the Israelites (and by extension, for believers in Christ) and the idea that God judges peoples with equity, then God's divine judgment and what he will do with evil is understood in a radical different light.
I still remember my preaching professor warned us not to preach God's judgment unless we preach it with tears on our cheeks and deep sadness in our hearts. This is probably pointing to the right direction of what God was experiencing when he judges. The ultimate example of this heart-breaking sadness and remorse for human sins is depicted on Calvary. What really happened on the Cross is a divine mystery. Without exhausting the richness of the event, it does point us in the direction of God's seriousness with sins (that somehow His sinless Son has to die) and the tremendous loss and sadness that God bore when he did judge.
By extension, God's judgment is logically necessary if we believe in the kingdom of God. If we believe that God rules history and that evil is not the final word in this world, then we have to believe that somehow hatred, unfairness, selfishness, greed, lust, violence, terror, etc will be brought to an end. The two World Wars that killed millions in this past century had brought disillusionment to the utopian notion that a perfect society is achievable by mere human efforts. The world is not going to heal itself of the problem of evil by mere technological advances, education, or the innate goodness of the human race. Evil will always exist unless God intervenes. As Christians, we have hope for a better world that is free of evil and sins precisely because we believe God will act to end evil once and for all.
And the word "equity" in this Psalm hits me hard. It says God judges peoples with equity. The Zimmerman case received national attention because racial inequality is still a present reality in the American experience. For Christians, the idea that God judges with equity means that we need to take justice, truth, and how we live them out seriously. Being Christians doesn't exempt us from being accountable to God on how we treat others who are from a different socio-economic and ethnic background.