what happen at the end of chapter 12 that is a terrible discovery?

There are three robbers on board, two of whom have tied up the third man. Apparently the bound man had threatened to turn them all in to the state. One of the robbers wants to kill him immediately, but the other man restrains him. The two men finally decide to kill their partner by leaving him on the boat and waiting until it sinks.

At this news, Huck scrambles back to rejoin Jim. Together they discover that their raft has come untied and floated away.