The king knows they committed their many sins in ignorance and that is one thing the Savior is willing to take into consideration. In the next verse he indicates that in spite of these terrible sins and many murders (which he mentions for the third time) the missionaries had taught them that God will forgive them because they had not been taught the gospel until now. Nevertheless, these sins were so heinous that they have resolved that to gain God's forgiveness by entering a covenant never to shed blood again even in self defense.