I have had to edit your question slightly to make it fit enotes regulations. We need to remember the context of this small episode to consider how Twain uses this section of the novel. The part you refer to comes in Chapter 35 of the novel, when King Arthur and Hank are travelling around the kingdom disguised so that they can see how just or unjust society is. Before the episode you refer to, King Arthur and Hank are seized and purchased by a slave trader who travels around with them in tow. One night they travel through a snow storm and many of the slaves perish. However, the slave trader seizes a woman who is fleeing her village because they wish to burn her because they say she is a witch and burns her at the stake so he can warm his slaves for free:
They fastened her to a post; they brought wood and piled it around her; they applied the torch while she shrieked and pleaded and strained her two young daughters to her breast; and our brute, with a heart solely for business, lashed us into position about the stake and warmed us into life and commercial value by the same fire which took away the innocent life of that poor harmless mother. That was the sort of master we had.
Thus this episode presents a comment on the brutality of the slave trade and their master in particular - note the reference about the slaves being warmed into "commercial value" by the fire that killed the mother. It implies that there is something deeply evil and wrong about the slave trade as a whole.