Literary: Mormon includes his personal reaction to the abominations of his people. This is not a required part of the narrative, but an expression of a man who cares for his people and is witnessing the ending of his world. When he gives this lament, it is in the form of a short antithetical parallel:
1 And wo is me because of their wickedness
2 for my heart has been filled with sorrow because of their wickedness, all my days;
3 nevertheless,
4 I know that I shall be lifted up at the last day.
The contrast is between line 2’s “they/wickedness” and line 4’s “I/lifted up.” Mormon is creating the ultimate contrast between a people who have departed from the way of the Lord and those few (of whom he is one) who continue to follow the Lord. While the ultimate mortal fate will be shared, the eternal fate will not.