What a lovely read:

"Emotional balance: The sniper must be able to calmly and deliberately kill targets that may not pose an immediate threat to him. It is much easier to kill in self-defense or in the defense of others than it is to kill without apparent provocation. The sniper must not be susceptible to emotions such as anxiety or remorse."

Source: FM 23-10, US Army Sniper Training Field Manual, page 1-4 on Personnel selection criteria (html or pdf).

An interesting read - if not exactly aligned with any career path ideas I might have...