It wasn't slavery that made it better, it was the colonization. Moreover, "better" is subjective - culture can take many forms.
The knowledge slaves learned when shipped back to Africa helped form slightly more intelligent civilizations.
Slaves that got to stay get to live in America free from disease, poverty (relatively speaking), starvation and dehydration.
So slavery both made Africa a slightly better place and Africans in america are living better than they would if they were still in africa.
If anything, the only downside about slavery and I can back this up with the FBI crime statistics is that black people are actually making this country worse. I mean, look at Detroit.