What does Isaiah 15:9 mean?

The suffering and grief of those in Moab is crushing. Those who have survived the slaughter of the foreign invaders as they wiped out one city after another are deep into mourning (Isaiah 15:1–3). Isaiah is revealing their future suffering in this prophecy about the coming judgment of God on Moab. This time of mourning and grief had not happened yet when the prophet was writing this oracle.

He now adds that the waters of Dimon are full of blood. "Dimon" may be a grim play on words for the city of Dibon in central Moab. "Dimon" sounds like the Hebrew word for "blood." The point is that so many people have been killed in the city of Dibon that the city's water supply is running with their blood. In less poetic terms, human corpses and other war debris have contaminated the water supply. It's a horrific picture, but it is not the end of Moab's suffering.

The Lord adds that He will bring on the city of Dibon even more suffering. Those who escape the initial slaughter will soon face another threat, poetically referred to as a "lion." It's unlikely actual lions will attack the remnant of refugees; Isaiah seems to use this as a metaphor for some other catastrophe. In either case, the Lord of Israel is taking full credit for the continued suffering of the people of Moab. This judgment is from Him.

This oracle was unlikely to have been written by Isaiah for the benefit of the Moabites. That is, this message is "about" the Moabites, but not "spoken to" them, directly. The point of all the oracles Isaiah has written was for the people of Judah to understand that the Lord alone is trustworthy to provide for and protect His people. Alliances with other nations cannot be relied upon for safety from harm. Only the Lord can give them the security they so desperately want.