But if doing evil(killing) is always the work of evil/amoral people can you actually have "good" soldiers? given that the work of a soldier at war involves killing, especially as modern mechanized armies use weapons that makes it nearly impossible to avoid collateral damage.

Im of cause using moral in the relative term here as following an "code of ethics" and acting in "the interest of the common good" as described in that code rather then simply acting from hedonistic/sadistic impulses.

The problem with absolute morality is that it's of absolutely no use to an rational actor dealing with an real world that don't conform to the idealized/simplified model that such moral codes tend to depend on for logical constituency.