Lets say there are 2 factions at war. One Evil and One Good.

That's a ludicrously strong assumption. Even the weaker assumption that good and evil are even well defined is still too strong for practical purposes.

There is simply no such thing as "good" or "evil" which can be canonically defined without reference to some expected value assigned to the human experience at a bare minimum. Any reasonable definitions of "good" and "evil" require at least one nontrivial, nonphysical, subjective value judgement to be fixed as part of the axioms of the ethics that define them.

Evil people can just ignore international laws and commit war crimes, Good people will have to abide by laws.

In my view, good people ignore international laws all the time, such as migrants crossing into countries illegally. Evil people often enforce international law...such as those who enforce laws banning migrants from crossing international borders. Notice that I've made the value judgement that borders are stupid [1]; there's no physical reason to indicate that migrants crossing borders have any moral relevance.

Country B: Half Good Half Evil

This requires the even stronger assumption that good and evil act like numbers, e.g. things you can add and get a predictable result, or take half of. Now I'm an engineer and math... enjoyer... so I get how convenient it would be to work with numbers. However, good and evil cannot be measured, and we have not found a mathematical model that adequately or unambiguously models the goodness or evilness of an act or person as a number or more generally a field. The law of unintended consequences suggests that approximating ethical decisions as operating on a number or number-like object doesn't yield reliable results in general.

The rest of your arguments rely on further stronger assumptions about how people "would" act. People are complicated, unpredictable, time-varying, and inconsistent. It is difficult to predict what people will do, and for this reason you need to supply additional facts to establish a cause for why people will follow your assumption. For your points to be valid, there needs to be either some historical evidence or at least a heuristic justification under much weaker assumptions when evidence is unavailable.

Said differently: your argument is fine (I think), but your premises aren't true in general, so your conclusions based on them cannot be true in general.

[1] Actually, "borders are stupid" is an unnecessarily strong assumption, because I could start with much weaker assumptions, such as "human beings are more important than the laws that govern them" and then deduce that borders are stupid from history and a minimal set of axioms. But this comment is getting long enough, and I honestly find ethics to be kinda dry.