A profit of at east one hundred percent was expected.

Vast wealth was obtained through the slave trade, and this money was reinvested in the developing industrial revolution.

Thereby the Africans unwittingly helped to finance the European industrial revolution which widened the technological gap between Africa and Europe.
The African slave was sometimes a criminal, but, more often than not, he was captured in battle.

As the slave trade grew and with it the need for more slaves, the number of these battles increased.

Clearly, many battles were being fought solely for the purpose of acquiring slaves who could then be sold to the European traders.