The book of Habakkuk is a compilation of the prophet's laments, not an accusation against Israel and its sin or a message to the people on God's behalf like some of the other prophetic books. Instead, Habakkuk questions God's goodness because he sees so much injustice, evil and tragedy in the world. He's also concerned because God plans to send Babylon, an intensely evil nation, to judge Israel.
Throughout the book, we see that Babylon is an example of any nation that exalts itself above God and practices injustice, violence and idolatry. In the end, God reminds Habakkuk and every generation that God will deal with evil. We can continue to love and trust His timing and plan as we remain faithful to Him.