What do you plot against the Lord?
He will make a complete end;
trouble will not rise up a second time.
That is the way God handles those who try to work against Him. They will not succeed. Their defeat will be complete and they will not live to trouble anybody a second time. God's justice and judgment are full. His purposes always prevail. No one may stand against Him.
With the prophet Nahum, the end of Ninevah is proclaimed. Jonah is a story about Ninevah's gracious moment of repentance granted by God. Nahum however is about Ninevah's final fall. Their wicked oppression and vile idolatry will never stand against God. God would make a final end of Ninevah.
So what should I do with this fact? I think it leads me to worship a strong God, not because His judgments or actions are vindictive, but because He is both gracious (Jonah) and firm (Ninevah) in His righteous actions toward humanity. And the God of both grace and justice came to earth in Jesus His Son to graciously save me while putting a final end to sin and death's hold on this world. Sin and death will not rise up a second time because Jesus has defeated them entirely through His death and resurrection.