they justified their conquest as a massive "holy war" to convert the aztec and inca to catholicism; if they instead murdered more than converted; they got the gold,land, and other minerals from the conquered peoples; as well as some slaves from the surviving poppulace; the cultural and religous societies the aztec and inca and others had did things the conquistadors and the countries they left from disliked; like a monotheistic noncatholicism religion and human sacrafice; the spanish also fought a bloody war to drive out the moslem north african decended rulers and also fought bloody wars against protestent europeans including some spanish; so the spanish conquistadors were used to fighting and destroying non catholic countries; and making plunder and slaves of the defeated after; while using their view of catholic and gentile cultural need to "civilize" and get "glory or credit for the conquest.