Cecil Rhodes was an extremely interesting man. I'll summarize why he was in South Africa by saying that he was one of the founders of the De Beers Diamond Company which eventually established a commanding position in the Diamond Fields of South Africa. After this, he set out to become a political figure in South Africa. South Africa, specifically the British Cape Colony, was at this time, in 1880, a Colony of Britain, with a white settler population and a large African population. Rhodes was an imperialist through and through, and wished to see the expansion of the British Empire "From Cape to Cairo".

To do this, he set about extracting mineral concessions from different African kingdoms in South Africa, while simultaneously pushing, through his friendly local Commissioners, for Britain to protect and control these areas as protectorates by signing related treaties, which also guaranteed the mining concessions he signed. His role in extending British control for his own benefit would make him controversial enough, but his treaties were often signed dishonestly. For instance, he convinced the King of Matabeleland, which would later become part of Southern Rhodesia, named of course for Rhodes, to give him mining concessions and promised him no more than ten white men would mine at the site promised. This was left out of the final treaty. He did this many times over throughout Africa, while also arranging for his British South Africa Company to obtain the contract for providing security for the mining concessions. In these areas, particularly Matabeleland, British settlers arrived quickly, and when the indigenous inhabitants rebelled against them these rebellions were put down violently by the company. Rhodesia would later become a settler colony much like South Africa.

In addition, Rhodes became the Prime Minister of the Cape Colony, passing a wide variety of discriminative acts against native South Africans, including depriving them of their lands and disenfranchising the vast majority of Africans in the colony. His aggression towards the Boer Republic of the Transvaal, through his support of the Jameson Raid, was what finally forced him to resign.

In summary, the man was a man who believed that the Anglo-Saxon race was destined to rule the world, and believed that the native Africans should be treated like children. While he was not the only architect of Apartheid, his role in establishing this discriminatory system which only ended in South Africa in 1994 and only ended in Southern Rhodesia (Now Zimbabwe) after a protracted civil war is bound to make him a controversial figure.

If anyone has any questions I'd be glad to answer them, I would also suggest reading Diamonds, Gold, and War: The British, the Boers, and the Making of South Africa by Martin Meredith.