The king of the Lamanites is a continuation of the dissenter Nephite dynasty begun by Tubaloth’s uncle. Amalickiah was the founder of the dynasty who attained the throne of the Lamanite hegemony by deception, described in Alma 47:18-19. Upon the death of Amalickiah, the throne passed to his brother Ammoron, and with the death of Ammoron to the son of Ammoron, Tubaloth.
Tubaloth has had a father and uncle die at the hands of the Nephites, from whom they had apostasized and against whom they had already developed strong negative feelings. Tubaloth was fated to have the same, and perhaps have those negative feelings intensified by the deaths of his father an uncle. It is no surprise that he wanted to find a way to go to war against the Nephites.
Variant: In both manuscripts and the 1830 edition, there is an additional phrase in verse 16. The original read:
“Therefore the king of the Lamanites whose name was Tubaloth, who was the son of Ammoron, now Tubaloth supposing that…”
The phrase “now Tubaloth” was removed from the 1837 edition to make the sentence read more smoothly. It would have been preferable to remove the initial “therefore.” Those are the two phrases in conflict, and the concept would have been clearer with a declarative sentence of Tubaloth’s position and genealogy, and then the comment about his actions rather than having the entire phrase try to be linked with the causal “therefore.” The phrase after the “not Tubaloth” provides the meaning for “therefore."