Gideon had come to slay the king, and now chooses not to. Of course the presence of the Lamanites is the fulcrum on which his choice moved from death to liberation, but why? The Lamanites posed a clear threat that Gideon wanted to meet. As a leader of men, he had a home and a land to protect.
Nevertheless, we may assume that with his previous intent, had be been able to quickly dispatch Noah he may have done so. As with the previous physical conflict with Noah, the king was apparently able to hold his own for at least a little while, and in this case would have held the more defensible ground, making Gideon's attack even more difficult.
Given a choice between a protracted delay while he killed the king, or the immediate ability to rally forces to meet a much larger challenge, Gideon chose the more pressing immediate threat of the Lamanites. This also suggests that Gideon was fairly firmly in control of the government at this time, so that he could presume to rally the people to their defense in spite of the freedom of Noah.