I always find this a very odd concept. An army's business is to fight. Fighting usually will involve killing someone.
Apart from that,
1: context does not tell us but an army will train its soldiers to shoot well enough to kill the enemy.
2: "operate a policy" - It is decided that "shoot to kill" is the way in which a situation will be contained, so all personnel are instructed that they must shoot to kill.
- For Teachers