Are you saying that Drake wasn't hung? I assume that had he been caught by the Spanish government he would have been hanged (but that is a different thing altogether).
Pirates are violent thieves (by definition). Privateers are violent thieves with government approval. Either way they are thieves commiting violent acts that would have been considered crimes had they been done by soldiers.
Drake had PR and he clearly knew how to manipulate British political power.
I don't know if you can defend Drake's slaver behavior even by the moral values of the time. He broke the laws of the time to capture slaves and bring them to the colonies.
I don't even know what you mean that "everyone was into slavery". That clearly was not true, in fact there was already an abolitionist movement.
He was cruel. He was a criminal. He was driven by greed and a lust for power.