The nations in this Psalm refer to the opponents of the author. They are described in different ways: in Psalm 9:3 my enemies,
Psalm 9:5 the nations,
Psalm 9:6 the enemy,
Psalm 9:13 those who hate me.
Here they are called the nations
again, and they have fallen into a pit that they dug—obviously it was meant for others, like the author. In Psalm 7:15 we read the same thing. As they fall into the pit they dug themselves, the author recognizes the actions of the Lord, the Judge (Psalm 9:4).
Next, the enemy gets stuck in a net. Parallel to the digging of a pit, which was meant for someone else, they themselves fall into it. They get stuck in it. The Lord is the righteous Judge (see Psalm 7:8). The Lord has made himself known. After all, he does justice. God intervenes in the lives of people. It can also be that the unbeliever perishes because of his own actions.
In the meantime, the author knows that not every sin is punished immediately. In this earthly life, justice does not always triumph. A lot of injustice remains unpunished.
Psalms such as these should not be disconnected from the final judgment that the Most High will execute one day. That judgment will come through the Son. In Athens, Paul said because he has fixed a day on which he will judge the world in righteousness by a man whom he has appointed; and of this he has given assurance to all by raising him from the dead
(Acts 17:31).
15 The nations have sunk in the pit that they made; in the net that they hid, their own foot has been caught.