From a modern context, this is a fairly simple situation. Lamoni’s father commands Lamoni to slay Ammon and not go to Middoni. To read this as only the request of an angry man is to miss much of the political consequences of this occasion. The father of Lamoni is not just a father, he is a king, and a king in a superior position.
This is not a moral choice, but a political choice that is being requested of Lamoni. Lamoni is being asked to declare his allegiance, either to his overlord and father, or to this Nephite. Refusal would be tantamount to breaking ties with his father and all of his father’s allies. Breaking such ties would create a situation where Lamoni was now in an antagonistic relationship with locations that has been friendly, and he would be giving up those political and economic connections without having replaced them. In an ancient society, this could be critical.
By refusing, Lamoni would be cutting his people off from economic and social benefits of the various allies in their current connections. In addition, he would be giving up a shared defensive strength to become an enemy to those people. He would be doing this as a single city, with no new alliance to replace the strength of the one he would be giving up. Lamoni had everything to lose, and virtually nothing to gain in the eyes of the world by refusing this demand of his father and overlord.