Things started going bad in the late 70-80's when the "Kin-Kiese" craze motived people from the country side to look for opprtunity in the big city (Mobutu's incompetence and in small part to song lyrics), and thus Kinshasa has been incredibly over-popluated for too long.
The population of Kinshasa is now 9.5 million, almost twice the size of the entire Congo-Brazza 5.2 million. If I remember correctly, the city was originally built to handle 1 million or less. No space, no reliable electricity, no functioning city-wide sanitation system, dirty water, and a very high child mortality rate