I don't have much to say about the first question because it's kind of a value judgment, but I know a little about the second.

There's a chance you're already familiar with the practice. If, for example, you were a subject of Queen Nzinga or a political rival who had somehow annoyed her, you would understand that if she were to sell you to the Portuguese, you would be in for a lifetime of servitude in a far-away continent.

If you're from the interior and were captured by a coastal group to be sold, you might think that the Europeans were going to eat you. There are multiple firsthand accounts of Africans interpreting forced baptisms on the coast as the first stage of a ritual cannibalism. Although it's outside of your time frame, Olauda Equiano's account is an example (how reliable it might be has been questioned recently).

The slave trade went on for a long time across a large geographical area, so these reactions are just two of many.

My sources are Professor Linda Heywood's lecture at the 2014 "1619 and the Making of America" conference at Hampton University, and also her book Central African, Atlantic Creoles, and the Foundation of America (I don't have it with me so I can't cite pages or anything but you should read it if you're interested).

Hope this helps at least a little bit.