What does Red Chief do which makes Bill and Sam wonder if they have made a mistake in kidnapping the boy in "The Ransom of Red Chief"?

Johnny Dorset, the son of wealthy banker Ebenezer Dorset, immediately attacks them; later, he terrorizes his captors even more.
When Sam and Bill ride up in a buggy and call to the boy, he throws a brick that catches Bill "neatly in the eye." They then have to wrestle Johnny to get him into the buggy:

That boy put up a fight like a welter-weight cinnamon bear; but, at last, we got him down in the bottom of the buggy and drove away.

After they transport the boy to the cave on the small mountain outside of Summit, Alabama, Johnny decides he wants to be called Red Chief. That evening, he talks incessantly and the men cannot get to sleep for hours. Sam then wakes at daybreak to hear Bill screaming as Red Chief attempts to scalp him.

I got the knife away from the kid and made him lie down again. But, from that moment, Bill’s spirit was broken.

Bill never really sleeps after this, as he feels he must keep an eye open all the time. Sam dozes some, but as the sun rises, he suddenly recalls that Red Chief declared that he was to be burned at the stake when the sun was rising. 

I wasn’t nervous or afraid; but I sat up and lit my pipe and leaned against a rock.

Soon, Sam goes to look down at the town and see whether a search for Johnny Dorset is occurring; however, there is nothing out of the ordinary happening. The kidnappers decide to write Mr. Dorset a ransom letter, reducing the original price by five hundred dollars. Ironically, the only way they can be rid of Red Chief is by paying Mr. Dorset to take the boy back.