I said slavery was banned in most European countries by 1820 and that slavery in the country's colonizing was pretty much gone. I did clarify that slavery in the us was banned about 20 years before but she insisted that the reason behind the conference was slavery and the us was one of the first to ban slavery. Am I an idiot what I have found indicates I'm right but im still not quite sure what caused the Berlin conference other than imperialism.
You are correct that slavery was already long abolished when the Europeans scrambled for Africa. Depending on the country, the exact date differs, but you can generally regard the mid 19th century (1848 in specific) as the moment that slavery (and serfdom, which was far more common on the European continent) was abolished.
The Berlin Conference can be seen as just another congress in the long 19th century system of congressional diplomacy and the balance of powers within Europe, which started with the congress of Vienna (1815), and which is generally regarded to have broken down completely with the start of the first world war (1914-1918).
Although there were major conflicts during the long 19th century, such as the Crimean war (1854) and the Franco-Prussian war (1870), there were no decades of conflict like the Revolutionary/Napoleonic wars (1792-1815). Disagreements and conflicts between the European Great Powers would mainly be solved by ambassadors of all great powers and some of the minor ones at a conference. Thus, the enormous costs and destructions of the Napoleonic wars would be avoided.
The Berlin Conference can be seen as a bit of an odd one, as it was an attempt at preventing conflicts instead of finding a compromise afterwards. It is mainly about trade, tariffs and deciding on the baseline of what is acceptable as "occupation" (so states couldn't just paint the map and claim it all.) So it was about Imperialism but in a way that would minimize conflict between the European powers. The second chapter, however, is about slavery and it tells us the following.
CHAPTER II
DECLARATION RELATIVE TO THE SLAVE TRADE
Article 9
Seeing that trading in slaves is forbidden in conformity with the principles of international law as recognized by the Signatory Powers, and seeing also that the operations, which, by sea or land, furnish slaves to trade, ought likewise to be regarded as forbidden, the Powers which do or shall exercise sovereign rights or influence in the territories forming the Conventional basin of the Congo declare that these territories may not serve as a market or means of transit for the trade in slaves, of whatever race they may be. Each of the Powers binds itself to employ all the means at its disposal for putting an end to this trade and for punishing those who engage in it.
So there is an explicit banning of slave trade already in the act of the conference, which would later be reinforced with the 1890 Brussels Conference Act (yay another congress!). In this act, slavery and slave trade gets (once more) banned. There are more regulations in the act, all with the aim of hampering the slave trade.