I would agree that "right" is highly subjective, and related to the beliefs of the observer.

This is a variation of two questions: "how do we know right from wrong?", and "how can we tell what is actually going on?". It's no good being able to determine right from wrong, if you don't have an accurate picture of the situation being evaluated. And it's not much good having an accurate picture of the situation, if you can't tell right from wrong. In wartime, typically both of these aspects get distorted: both sides are thinking in terms of us-vs-them, identifying with their beliefs and group membership, and that distorts the moral judgment. In addition, both sides are engaging in obfuscation of the facts surrounding the conflict, so that makes it difficult to establish a clear view of the situation. As a practical matter, both sides are wrong in most conflicts, and only the details differ. One cool thing about morality is that there are generally more ways to be wrong than there are to be right, and that asymmetry means you can have a lot of villains opposing each other, with no hero in sight. Typically the hero is avoiding all hostilities and attempting to assuage the suffering of the innocent victims... providing medical care for the orphans, food for the refugees, comfort to the bereaved, and hope to the hopeless. It rarely gets much better than that.