...We thought we'd rip that Band-Aid off fast for any of you out there attached to Scout's older brother.
Early on in Watchman, we learn that Jem and Scout's mother died of a heart defect. She passed that defect on to Jem, and one day he keeled over, dead by the snap of fate's fingers.
In one flashback, Jem sweetly says he'll take care of Scout. In a way, we're kind of glad Jem is dead (does that make us terrible people?), since the rest of the Finch family is so despicable. By dying, Jem's legacy remains undisturbed.