Lehi parallels his statement of general concern for their welfare with the admission that there have been times of serious sorrow and contemplation. While the wayward family members surely must have understood to some degree the strain they placed on Lehi, it is still somewhat surprising that Lehi so openly discusses with them the depths of his fears for them. This is not simply a lecture, but a baring of the soul - a glimpse into the heart-worn soul of a loving father struggling to raise all of his children in righteousness.
Lehi makes sure that these children know that the wrath of God would fall on them because of the hardness of their hearts, and not for any other reason. It is important to understand that the Lord has infinite patience, but it is we who will shut him out when our hearts are past feeling and we no longer listen for the soft voice of the Lord but rather deny it entirely.
Verse 18 is the second half of the concern of Lehi, and probably should be read with verse 17 always, as it completes that thought. Lehi presents his concern for his sons as two different fears. In the first they are cut off completely. In the second they are cursed, and the cursing includes wars and famines and clearly continues for years.
In the first condemnation, Lehi is concerned for the specific spiritual welfare of these particular individuals. It is their souls that might be cut off, it is for those individuals that Lehi fears. The second half of Lehi's "fear" is that the actions of these will precipitate a cursing upon their descendants. Thus the caution has two parts because it is directed at different generations, not because the Lord had an "either - or" option for their condemnation. The specifics of the second phrase and the subsequent history of the Lamanites suggests that it was this second condemnation that fell upon them, and that Lehi had perhaps seen that future in a vision so that he could so well describe it.