Five years ago, a masked killer brutally murdered a group of friends. Since then, [character A], the lone survivor, has struggled to reclaim a shattered life. Wracked with guilt and paranoia, [Character A] leads a depressingly lonely existence until [Character B] befriends [Character A] and attempts to integrate [Character A] into the Adventuring Party.
But just when [Character A] might be ready to start a new life, [Character A]'s past comes back to haunt them all.
Out damn spot
How is it that PCs can go off and murder hundreds without being psychologically changed by the experience? I suggest that every life taken requires a charisma check with a cumulative -1 penalty for every life taken. If the PC is unable to cope emotionally, it should cost one point of charisma or an alignment shift toward chaotic and evil. When charisma reaches three a multiple personality emerges with an alternate alignment (and even class).