As we discover fairly late in the novel, Charlie experienced some trauma back in the day that explains a lot about his behavior towards others. A cold, unsentimental man, Charlie keeps his emotional distance from the human race, a very handy characteristic to have if you're a hired killer.

The traumatic event in question involved Charlie's killing his old man. It happened one day when Mr. Sisters was having a massive row with his wife, Charlie and Eli's mother. The argument quickly descended into violence as Mr. Sisters broke out the window with his fist and hit his wife on the arm with an ax handle.

Believing that his old man had gone crazy, Charlie shot him dead with his rifle. His father had always made fun of Charlie's gun, calling it a “pea shooter.” But he certainly wasn't making any jokes about it when Charlie pulled the trigger and sent him on a way-way trip to eternity.

Since then, it's been noticeable that Charlie appears somewhat cold to other people. This is almost certainly a direct result of that fateful day when he shot and killed his father. After an experience like that, it's no wonder that he stands apart from other people. In the world inhabited by Eli and Charlie, violent death may be common, but the killing of a parent is still a rarity.