Do not enter the gate of my people
in the day of their calamity;
do not gloat over his disaster
in the day of his calamity;
do not loot his wealth
in the day of his calamity.
Obadiah is the shortest of the "minor" prophets with a message of God's impending judgment on Edom. Esau's descendents would suffer and be wiped out over time. One of the chief reasons for their coming due justice was their actions and attitudes toward Judah and Israel when they were led captive. Edom rejoiced at Judah's fall.
Edom gloated over the fall of Jerusalem like they were the conquerors. They were not humbled by the loss to a neighbor, but rejoiced over the destruction, death, and sorrow. They hated their enemies with glee. And this was one reason God would also humble Edom.
We should never rejoice in calamity. Human suffering is always a reminder of our fallenness and the consequences of sin, whether it be due to an act of man or a natural disaster. People who seek God's heart should never rejoice at the fall of an enemy. It's serious business when God's justice is at work. And that is the point of just why Obadiah was led to remind Edom not to make light of another's disaster.