I would think he should be called "Dictator" or are we straying from the one word examples? It is not pedantic to call Columbus an explorer. It wouldn't be unreasonable to use other single words, maybe "Colonizer" but he wasn't really that either. Most of what people are suggesting are highly colored by emotion. I'm not seeing the controversy in "Explorer" unless you have a very childish/naive connotation with that word to mean only fun, exciting, and adventurous. Exploring can be evil.