I am often baffled by this reasoning: A attacks B. A colonizes, kills, occupies, humiliates, etc. B. B swears to destroy A. All of a sudden, it becomes legitima

I am often baffled by this reasoning: A attacks B. A colonizes, kills, occupies, humiliates, etc. B. B swears to destroy A. All of a sudden, it becomes legitimate for A to commit all those crimes cause "hypothetically", B would do worse if they could.