All disqualifications would concern things that would compromise the mission.
The number one thing that would compromise a mission would be for the marksman to get caught or killed by the enemy. If the candidate is not resourceful enough to avoid detection, then he would likely fail at performing the mission.
Other things that might compromise the mission would be:
1. Physical problems;
2. Mental or emotional problems; i.e. an overly gung-ho individual would compromise the mission as surely as a person who might "freeze"