African leaders participated in the slave trade for a number of reasons. Pan-Africanism did not yet exist, so there were no strong bonds among the myriad tribes of Africa. African leaders chose, not surprisingly, to participate in the lucrative triangular trade among Africa, the West Indies, and North America.
Slavery and its concomitant slave trade existed throughout recorded history. The Greeks and the Romans practiced slavery. Therefore, the participation of African leaders was not unusual. In fact, it would have been highly surprising had the African leaders chosen not to participate in the slave trade.
The African leaders did not have much of a choice. European explorers of the Age of Discovery were ruthless and brutal men. They were motivated by "gold, glory, and God." It was thought that the Bible condoned slavery, so Christians of the era both enslaved and proselytized those they met during their voyages. Whites and Indians were not good slaves, so Europeans focused on Africa for its supply of forced laborers. Also, an African chieftain who was brave enough to say "no" to Europeans risked enslavement of his own tribe or a disastrous war will well-equipped Europeans.
Third, the African slavers did not know the people they enslaved. They captured distant peoples from the interior of the vast African continent and forced them to march to the coast. Perhaps this fact made it easier for the slavers.