What are the cons of colonization?

Entire books have been written about this. In a nutshell, colonization derailed societies; destroyed existing cultures, religions and systems of government; and gave colonized peoples a persistent inferiority complex. The colonial powers did have some good intentions, like providing education and technology to those who did not have it, but too often it was accompanied by subjugation that denied people their rights and limited them to menial jobs providing raw materials that fueled the empire's economy. Former colonies in Africa, Asia and Latin America are among the world's poorest countries, and are only now beginning to recover from their colonial past.

It should be noted that some former colonies, like Australia, Canada and the United States, are doing quite well. This is largely because they were settled by Europeans, who had money and built a society that could support large numbers of Europeans. In other, later colonies, fewer Europeans moved in, so less infrastructure was built (because Africans, for example, didn't matter very much). Upon independence, a country inherited a government from its colonizer; some of these were very developed, like in the United States, while most were rudimentary in the extreme, like the Congo.