To be 'Gun Shy' is a military shooting term. It is the blinking of one's master eye (the eye one take's aim with behind the telescopic sight or 'naked' sights of a projectile weapon)
simultaneously as the trigger is pulled. This involuntary action is a sign that one is as yet unable to take responsability for a round leaving ones weapon and killing someone. This is obviously
unacceptable in a
professional shooter, especially a sniper. One should never take eyes off target, and a closed eye is an eye that does not see a threat.
But aversion to killing is natural. Thankfully.
In most soldiers the condition of being 'gun shy' is capable of being overcome with the rationale that the enemy is a willing combatant and is a lethal threat to you, your comrades, and any innocent third-parties, and it is simply a case of 'you or them'.