Which was the most important reason as to why slavery was justified?

Slavery was justified by saying that it was 'natural' or 'beneficial' and that it was part of gods plan and to interfere was wrong. Also, people thought that slavery taught people how to live their lives if they thought they lacked the ability to. Mainly also because it was legal, however this is not a very good argument because it can be legal but unethical. Abolishing slavery would threaten the structure of society is another good point.

I suppose to gain money, grow the economy. Most likely why in the beginning at least.