Why did slave trade end into colonisation of Africa?
From the very start of the slavery period in South Africa, people who were categorized as slaves started to run away due to overwork, ill-treatment, and a desire to live their lives by themselves - as a free person.
During the beginning of the 19th century, the concept of human rights was gaining a lot of attention. During the same period, changes in the economic system in Western Europe made people question the practice of slavery. Also, note that Cape Town (current name, the capital city of South Africa) was occupied by the British in 1806 and it officially became a British colony in 1814.
The British due to the growing concern introduced a new set of law called Amelioration laws for the welfare of slaves in the Cape. This new set of laws became the foundation for eradication of slavery going forward. The British government appointed a slave guardian who was made responsible for enforcing these laws. This resulted in improved lives of these slaves after 1807. Some of the masters (who used to keep slaves) still continued to ill-treat their slaves by ignoring the reform laws and still giving cruel punishment. The slaves were then expected to report any such incidents to the slave protector appointed by the colonial government, who would then be charged per the rules.
The end of slavery at the Cape was not due to internal pressure but from a decision from outside. In 1807 the British government banned the slave trade to all her colonies, including the Cape. This meant that no more slaves (from any destination) could be sent to work in the Cape. However, those who were already in the Cape continued to work as slaves until 1834 when all slaves in the British Empire were to be set free (emancipated). Many of the slaves chose to remain on with their owners while some started a new life in and around Cape Town working as tradesmen. Gradually these people became absorbed into the Cape community.