It's not about (justified) revenge, it's about security. 1500's Spain was as dangerous to the natives of the Americas as the Romans were to the Carthaginians 20

It's not about (justified) revenge, it's about security. 1500's Spain was as dangerous to the natives of the Americas as the Romans were to the Carthaginians 2000 years earlier. If some society didn't have qualms with attempting planetary scale biocide, who's to say their genetically practically identical successors won't do the same?