The deaths of Lincoln's mother and sons. Both in the book and in the movie.
Examples from the book include:
The specific events surrounding the deaths of each of Lincoln's family members from the vampiric disease. Imagine your most loved ones being forced to imbibe a creature's cursed blood, then helplessly watching them lose their health, their sanity, and at last their lives.
A section from the poem "A Suicide's Soliloquy" is reprinted in the book. The circumstances surrounding the poem are accurate to reality; it appeared in the Sangamon Journal shortly after Ann's untimely death, and it's often attributed to Lincoln. Also featured is the entirety of the poem "Little Eddie", co-written by Abe and Mary Todd.
Examples from the movie include:
The death of Henry's wife, especially since Henry was Forced to Watch as Adam drained her dry.