The modern world has changed considerably from the ancient world depicted here. Not only is Zerahemnah unwilling to take an oath that he knows he will not keep, but when Moroni hears that Zerahemnah will not agree to Moroni’s terms, he gives him back his weapons of war. Moroni had rearmed his enemy.
In verse 11, Moroni says: “I cannot recall the words which I have spoken.” In modern usage, we tend to read this as “I cannot remember the words which I have spoken.” That is clearly not the meaning in this case. Here, the word recall is used in the sense of bringing them back, or undoing them. It is the sense we use when a recall is made for a product. It is to be brought back. Moroni’s words were just as much an oath as he could expect from Zerahemnah. Just because Zerahemnah would not take an oath (Moroni knew that Zerahemnah wouldn’t abide by it), Moroni would not change the offered oath simply because it was not accepted. Therefore, Moroni reiterates that if they do not accept the oath, they will be slaughtered.