Is your argument "genocide was good for them because maybe someone would have genocided them worse"?? The issue is not so much that they took over Australia. Yes, that was indeed going to happen. It's their treatment of the indegenous populations. Taking over countries is bad, but there's taking over countries with respect to the indegenous populations and their culture and then there's taking over countries while genociding them, or enslaving them or stealing all they have.