Both Uses of

recoil
in
The Shack

Mack recoiled at the thought of his children being watched; being targets.†
Chpt 4. *

recoiled = moved back suddenly

Definition:

to move backward suddenly (sometimes figuratively)

especially:

the backward jerk of a gun or cannon when it is fired
when a person flinches (suddenly draws back) from someone or something, as with fear, disgust, or pain
when a person is emotionally repulsed, as by disgust
when something intended to go in one direction figuratively falls back in the opposite direction; for example, a story told to hurt someone that comes back to hurt the teller