Abolishing Slavery Aided the Rise of Europe's Colonial Empires

No one seriously defends slavery today, but the way in which we view this historical phenomenon has become a cliched caricature. Was it, at least in Africa, part of a much more complex socio-economic picture, one in which the natives played many key roles? When Europeans unilaterally decided to abolish the institution in the early part of the 19th century, what were the real outcomes on African society, and are these in any way linked to the sudden spread of European colonisation in the latter part of the 19th century?