For example, Rhodesian white, freshly independent government was succeeded by Mugabe, who began to train army brigade just to kill Ndebele people. Same, or worse, results could be seen in Uganda, Congo, Zaire, Malawi... What were the common factors, and could it have been prevented?
This is an incredibly complex topic, but there are two major (but by no means the only) factors.
1.) Colonial boundaries were drawn with no regards to the ethnic, tribal, or religious groups of the natives; mutually antagonistic groups often found themselves stuck within the same country (e.g., Christianity and Islam Nigeria), while other groups ended up divided by borders.
2.) A common colonial method of administration was to favor a minority group within the colony at the expense of a majority group; the minority's newfound privileges ensured loyalty to the colonial government. This is the major cause of the Rwandan Genocide.
An excellent (though increasingly dated) book on this topic, and much else in Africa is John Reader's "Africa: A Biography of a Continent."