Any population can go from oppressor to oppressed, or vice versa, so long as people still make the mistake of thinking that revenge and force actually solve anything instead of just perpetuating the problems. The mistake my father was reflecting on was assuming that there's a fundamental difference in nature between your own group and whatever other group you see as the enemy, assuming that their psychology is somehow fundamentally different from your own. Ultimately you get better results if you step back and recognize that all populations would react in largely the same manner if they felt -- justifiably or not -- that they were being victimized.
Sorrry, switching topics too swiftly, and not being clear. The Japanese government was an imperialist aggressor. But it was not necessary to purposefully inflict massive civilian casualties to force an unconditional surrender. As in, if, after the Zionist state is overthrown, the new Palestinian state begins a violent oppression of all the Jewish inhabitants? If that's what's meant, then the point is likely true enough and unobjectionable. It just seems utterly irrelelvant.