Before we even get to a description of the violence perpetrated by white slavers, these lines foreshadow the oncoming disaster. Check out the adjectives: "ungentled" and "bold" aren't exactly a warm and fuzzy combination, are they? That's precisely the unsettling feeling Angelou wants to create, so that by the time we get to the actual violence, we're already on the edge of our seats.
took her young daughters sold her strong sons (13-14)
Notice how both of these lines start with verbs? It breaks each line into a short, violent action – "took" and "sold" seem almost like wounds that erupt onto each new line. The absence of much other description makes these actions stand out in stark relief.
churched her with Jesus (15)
For Angelou, Jesus is an imposition upon cultures that already had their own well-developed belief systems. The fact that this line starts with a verb (like all the others in the middle of the second stanza) allows the speaker to lump religious conversion in with the forms of violence perpetrated by slavers.
bled her with guns (16)
The funny thing is, it's not actually Africa who is bleeding; it's the people of Africa. Angelou's speaker uses the continent as a shorthand for all of its inhabitants here.
her history slain (23)
Slavery may be (almost) over, but the loss of history is something that continues to haunt colonized peoples the world over. Ripping people away from their cultures and communities makes it hard for them to maintain a sense of their past. In many ways, this is a form of violence just as important as physical violence.
remember her pain remember the losses her screams loud and vain (19-21)
Now suffering opens the door to transformative experiences as the speaker urges us (the readers) to use suffering as a rallying cry to fight for a better future.