Why was the colonization of Africa a bad thing?

This is not a question that can be fully answered here. Colonialism was born out of racist ideologies and the idea that the people of Africa were not entitled to keep either the land in which they were born or the natural resources that land contained. European countries warred with each other to conquer, divide, and mine Africa for its resources, without considering the long-term effects on native Africans. These effects can still be seen in Africa today and have left it a continent disconnected from its own history, divided by borders drawn on a map by Europeans, which separated tribes from each other and overrode the boundaries which had previously existed for centuries.
Today, Africa has a significant amount of impoverished and war-torn areas. Its natural resources have been stripped away, and its native cultures have been worn down by centuries of foreign values. Because the European colonizers sought to control their colonized peoples, they propagated an idea of African culture as inferior and even nonexistent, such that many Africans today still labor under the false impression that Africa—which has had its own great civilizations, including Ancient Egypt—has contributed nothing to the world. Africans can sometimes suffer from a loss of identity. In South Africa, Apartheid was an overt demonstration of the racism that drove colonial expansion, with black Africans demeaned, diminished politically, and stripped of their lands in favor of whites.
During the colonizing years, thousands upon thousands of Africans were massacred by European colonizers. This may no longer be happening, but the damaging effects of colonialism—the imposition of foreign ideologies, which held that black people were inferior and that they must adhere to a social structure not their own—are still strongly felt in Africa today.