Nephi sees, in verse 19, the final destruction of his people. He has been describing events in his vision, and then we have an interesting break with verses 16 through 18. Rather than describe what will happen, the angel reverts to Nephi’s dream and picks out elements.
The angel emphasized the fountain of filthy water and the depths of hell. Even in the presence of the tree and the fruit, there is the world and the opposite opportunity. On the one hand, there are the fruit of the tree and joy. On the other, the depths of hell, and misery.
Thus, the mists of darkness can blind one’s way, and many are lead to broad roads of the devil rather than the narrow path to the tree. The temptations of the secular world lead to pride, and that pride in the world becomes the terrible gulf that separates the large and spacious building from the Messiah, who is the Lamb of God. Without saying so, the large and spacious building is across the gulf from the tree.
Nephi sees that his people are destroyed. He also sees that God did not default on his promise. The promise was protection upon righteousness, and Nephi sees that his people succumbed to the pride of the world and found themselves with a gulf between themselves and the narrow path, or the way of righteousness.