The answer to this question can be found in Chapter 14, which is where Huck and Jim have their debate about kings and royalty, and Huck tries, without success, to impress on Jim what an important person King Solomon was. In particular, Huck talks about the famous dispute between the two women claiming to be the mother of the child where Solomon finds out who the true mother is by telling both women to take hold of one hand of the child and pull until the child split into two. The woman who refused to do this was therefore shown to be the true mother of the child. Jim, however, refuses to understand the subtlety of this story and merely focuses on his command to rip the child into two:
Now I want to ast you: what's de use er dat half a bill?--can't buy noth'n wid it. En what use is a half a chile? I wouldn' give a dern fro a million un um.
This is why Jim thinks King Solomon is a bad person, because he only focuses on the part of the story where Solomon orders the child to be ripped into two and not on the actual reason for this command and the purpose behind it. Jim goes on to argue that because Solomon had lots of wives, he had lots of children, and because of this, it didn't matter to him if one got torn into two, which again shows him to have no conscience. In spite of all of his efforts of persuasion, Huck is not able to make Jim see the truth, and he is left thinking that Jim is very stubborn.