The king confesses to stealing the money and putting it in the coffin while the duke is strangling him. If the king continues to deny stealing the money, the duke may kill him.
Just before the king confesses, Huck narrates that he "began to gurgle". This suggests that the duke, who had stopped attacking the king, has begun again in his physical assault on the king.
In order to end the argument and save his life, the king relents and admits that he was the one who stole the money.
The scuffle ends with the duke saying, "If you ever deny it again I'll drown you."
Huck feels "more easier" than he was feeling before knowing now that he is not suspected of stealing the money.