No, that's actually not what happened. First of all, they did not take over Congo because Congo never existed before Leopold drew it up on a map on a rainy Saturday afternoon together with Stanley. Also, the country of Belgium did not take over Congo, it was a private enterprise by king Leopold II, although the government later took it over from him after reports of atrocities started to trickle through to western Europe. Third, they didn't make people slaves. The slave traders operating in Congo were mostly African-Arabic. The Belgians levied taxes on the population. This was first collected in rubber, and the colonist but also the Congolese army committed atrocities such as the cutting off of hands, murder of village elders, and so on to maximize production. Many more lives were claimed by the sleeping disease. Later on they switched to taxing in money which meant that many young men had to work in the mines to earn money for their village, often under really bad conditions. The wars, dictatorships, and genocides started after the independence.
Don't get me wrong, I'm not making excuses here or anything, but it irks me when people reduce the complex history of a beautiful country to an overly simplified and plainly false representation of the facts.