Mosiah 18:32-35; Mosiah 19:1-12

32  But behold, it came to pass that the king, having discovered a movement among the people, sent his servants to watch them.  Therefore on the day that they were assembling themselves together to hear the word of the Lord they were discovered unto the king.
33  And now the king said that Alma was stirring up the people to rebellion against him; therefore he sent his army to destroy them.
34  And it came to pass that Alma and the people of the Lord were apprised of the coming of the king's army; therefore they took their tents and their families and departed into the wilderness.
35  And they were in number about four hundred and fifty souls.

Mosiah 18:32-35 (Emphasis mine)

Things were going well for Alma(1) and his followers.  Then, they were discovered by the King.

After everything that happened, Noah knew he had to get rid of Alma(1) and his followers.  They were converted either by Abinadi or his teachings through Alma(1).  Having felt the words of Abinadi were a major threat to him, he knew he had to get rid of these people to be secure.  His response was to send an army and have the army remove the problem.

Somehow, they were warned the King's army was coming.  As a result, they departed and went into the wilderness to escape the King. 

Chapter 19

Gideon seeks to slay King Noah—The Lamanites invade the land—King Noah suffers death by fire—Limhi rules as a tributary monarch. About 145–121 B.C.

1  AND it came to pass that the army of the king returned, having searched in vain for the people of the Lord.
2  And now behold, the forces of the king were small, having been reduced, and there began to be a division among the remainder of the people.
3  And the lesser part began to breathe out threatenings against the king, and there began to be a great contention among them.

Mosiah 19:1-3 (Emphasis mine) 

We return to the kingdom of Noah in Chapter 19.  The (small) army returned having failed to find Alma(1)'s group.  The forces of Noah were diminished.  Mormon doesn't tells us why it's smaller.  Could it have been the loss of people to Alma(1)?  I would think it didn't help Noah's situation.

After the return of the army, we are told the people are unhappy.  It's getting so bad, that Noah's status as king is being threatened.  Things are not going very well. 

4  And now there was a man among them whose name was Gideon, and he being a strong man and an enemy to the king, therefore he drew his sword, and swore in his wrath that he would slay the king.
5  And it came to pass that he fought with the king; and when the king saw that he was about to overpower him, he fled and ran and got upon the tower which was near the temple.

6  And Gideon pursued after him and was about to get upon the tower to slay the king, and the king cast his eyes round about towards the land of Shemlon, and behold, the army of the Lamanites were within the borders of the land.
7  And now the king cried out in the anguish of his soul, saying: Gideon, spare me, for the Lamanites are upon us, and they will destroy us; yea, they will destroy my people.
8  And now the king was not so much concerned about his people as he was about his own life; nevertheless, Gideon did spare his life.

Mosiah 19:4-8 (Emphasis mine) 

Things were so bad, Gideon decided the king must die.  He fights with the king, begins to win, and Noah ran to a tower to save his life.  Gideon followed and was about to kill him, when Noah saw a Lamanite army had entered the land of his kingdom.

Noah pleaded for his life and the lives of his people.  Gideon spared him.

We see exactly what type of person Noah was.  When he was sitting safely on his throne, surrounded by his court and supporters, he was a very brave man.  But, when he stood alone, Noah was a coward.  He didn't care about anything but saving himself. 

25  For whosoever will save his life shall lose it: and whosoever will lose his life for my sake shall find it.
26  For what is a man profited, if he shall gain the whole world, and lose his own soul?  or what shall a man give in exchange for his soul?

Matthew 16:25-26 (Emphasis mine)

9  And the king commanded the people that they should flee before the Lamanites, and he himself did go before them, and they did flee into the wilderness, with their women and their children.
10  And it came to pass that the Lamanites did pursue them, and did overtake them, and began to slay them.
11  Now it came to pass that the king commanded them that all the men should leave their wives and their children, and flee before the Lamanites.
12  Now there were many that would not leave them, but had rather stay and perish with them.  And the rest left their wives and their children and fled.

Mosiah 19:9-12 (Emphasis mine)

We read earlier that Noah didn't have much of an army (19:2). A weak army, faced with an overwhelming Lamanite army, would be of little use to protect the people.  So, Noah told his people to run, and he was gone before them.  

There were actually men who ran and left their families behind to face whatever fate awaited them.  I can't even imagine what manner of man would do this.  

Hugh Nibley talks about Noah and the type of man he was.

"Here's a comment on the character of [Noah] (sic) in verse 8. The king wasn't really interested in his people—it was his own life he wanted to save. That reflects on his character. You notice [Noah's] (sic) greatest sin was not lechery or luxury; it was his meanness. He was a mean, petty person. He had no feelings for anybody else but a great deal for himself. It's characteristic here of people with great power ... These are interesting human relationships that go on in the Book of Mormon. And then what did he do? He said the king was concerned about his own life; that was what really concerned him. He had no appetite for fighting, notice. He had lost his nerve along with most of the army. He panicked and said, we'll clear out of town. He ordered a general evacuation—a silly thing to do. It shows again that he's not really a strong character at all. He did it again when they go a little further on. He ordered his soldiers with himself to leave the women and children and hightail it to save themselves. Then when he got out with them, he fled with just a company of priests to get away from them. This is the kind of man he was, and we're going to see this all through here. Nice reflection on King Noah ... He went out with his court, and especially with his priests, his immediate accomplices. They accompanied him deeper and deeper into the woods while he left more and more of the others farther and farther behind." (Emphasis mine)

Lecture 37: Mosiah 19-20, Hugh W. Nibley, Maxwell Institute, accessed July 31, 2011.

There were many who stayed with their families and ignored the king.  They were ready to face the Lamanites and whatever would happen to them.