Arm manufacturers not only build weapons with the intent to kill, but profit from their sale. While there’s nothing wrong with protecting one’s nation or family, do you really think it should be a multi-billion dollar business?
ISIS rose to power specifically because the “Just War” doctrine wasn’t followed. Remember, one of the required elements is that the war cannot result in a greater evil, but that’s just what happened. The removal of Saddam Hussein under the Bush administration and later our support (albeit indirectly) of the Syrian rebels. Many modern wars, at least concerning Western involvement in them, directly lead to the rise of groups like ISIS.