[azernik]

a) war a very unforgiving environment - bad policies become apparent very quickly under pressure

b) the things you are against ("kill people's friends and families and not expect them to become terrorists") are often policies not under military control. If the government doesn't want to spend the money or annoy its allies enough to solve the root causes, there's not much a soldier on the ground can do except mitigate the damage (and yes, that often includes actions that perpetuate a bad status quo)