The reason for the mutants slaughter was because because they attacked first, most were like the signless but violent, they were the reason the caste was created, many eons ago there was no hemospectrum or pailing drones, they were just trolls, it was peaceful until the hot headed iron bloods thought they could take over all the castes, thus the great alternian war started, The highbloods won, and declared the Imperial Radiation ruler, she created the hemospectrum and who was in charge of the land, the purple bloods. It was peaceful, the mutant bloods gone, but then the Condescension came to rule, making pailing drones and making the lowbloods pay more taxes than the highbloods, the Grand Highblood came into rule of the land a few sweeps after, and then the Signless was born, preaching his words and starting riots, and the revolution. The war was brought all over again by a mutant, that's why there culled on sight.