The Righteous Should Mourn
Seeing that we are all God's creation, one is just the same as the other before Him. He rejoices and wants to rejoice over all of us--His creation. But we must be pleasing to God by the way we live--in the things we say and do, in our activities and actions. However, life does not happen that way. Some have been able to live a life of faith and obedience and others do not. And so God determines to execute His Word concerning the punishment of evil and wickedness while rewarding righteousness and obedience. Although there are many things in-between, nothing stops God from remaining true to His Word to punish and to reward.
God says in His Word that He's not interested in the death of the sinner, but that they should change their ways to start to act and live like His own children would (Ezekiel 18:32; 33:11). The reason the righteous will mourn when they see fellow human beings facing God's judgment is that one or some of us--fellow creation of God--just met a terrible fate. It shows something is fundamentally wrong with our world because God created man with a grand purpose in mind, and which does not include becoming an agent of the devil and devilish acts and behavior.
In the constant feuds and sometimes very strong opposition that happens among individuals and groups, when one side is struck by any form of tragedy, the other side rejoices. But not so with the righteous. For the one who understands the true nature of God regarding how He views human beings, when the wicked are destroyed for even things they have done wrong, the God-loving and God-fearing should mourn rather than find it a cause to rejoice. The reason for this is that God Himself does not rejoice at the destruction of the wicked, He would rather they change their ways so that they can become useful instruments for the causes and purposes of God. The other side of the coin is to stand in opposition to God's truth, working and moving against God's plan and purposes.
The faithful understand that we live by grace through faith (Ephesians 2:8 ) and those who have not been able to find this way of life are in constant danger of moving and acting against God, thereby putting themselves in direct opposition against God's will and truth. This does not call for rejoicing but for mourning, because without fail, the opposition will be destroyed.