I still don’t see any disconnect there between unethical and sinful. The pacifist and the soldier can both be making an ethical choice, despite their outcomes being different. So it can be that both their choices are acceptable to God (i.e., not sinful).
There is an acceptable range of “ethical choices”. It often depends on the motivations underlying a choice whether something is ethical or not. For example, if one man decides to be a pacifist because of laziness and another man decides to be a soldier because of hatred, both choices become unethical. This makes such choices unacceptable to God (i.e., sinful).
For me, immorality only has meaning within the context of accountability. So very young children (or animals, for that matter) who behave in manipulative ways are still not behaving immorally, although projecting our own accountability on them can give us the impression of immorality. Without accountability, there is no sinful, immoral or unethical behavior.
I think that all of humanity has been accountable before God ever since they could rightfully be called “human” (whenever that was). Bearing the image of God, no human being can behave immoral without it being sinful or unethical without being sinful. God holds a person accountable for their deeds. Truly unethical and immoral deeds are always sinful, as far as I can see.
I honestly don’t see how morality or ethics can be separated from our personal accountability before God. I believe in true morality and true ethics, even though there are ranges of choices and motivations that are acceptable.