I have searched the internet for a while, yet I can't find a brief comprehensive article on the topic. The wikipedia article doesn't really spell out how the Scramble took place and how it was justified.
Q) How did they justify the conquest of Africa? Did they simply declare: "This area is mine" and started conquering? Or did they systematically come up with worthwhile casus belli in order to keep the support of their subjects at their home country?
Land, riches, and rivalry so for instance Britain took some land and then France felt they deserved some too. So then Britain felt they should take a bit more... etc. So they believed that this was fair, because it's what the other one did, so why couldn't they do it.
As for moral reasons, many did believe that they would genuinely help Africa. The thought that they could help the countries be strong and give them civilisation and european technology.
edit check out the treaty they signed on section Africa off, a lot of the language avoids colonisation.