It is not socially acceptable to help free a slave.  It would go against the morals that Tom was taught. 

Twain is attacking prejudices against the black slaves.  It reinforces the theme of racism against slaves.

Huck believes he is going to go to hell for saving a person.  Twain is subtlely attacking slavery.  Huck has to think a

There is a big frenzy when the gold is found in the coffin.  Huck leaves during the frenzy.  No one notices him leave.

Huck decides to save Jim.

Huck told Mary Jane the con men's plan.  He does not want to be associated with the con men anymore. 

Huck thinks Mary Jane might show emotion on her face that reveals what she knows.  Huck does not want the con men to know that someone knows their plan.

The Wilks girls do not like that the slave family was sepperated

The duke and king only invite men to the show.  Humans want the things that are forbidden to them.  The men think that the show is going to be promiscuous. 

Twain included the circus as a comparison to the duke and king's entertainment.  The circus showed how not smart Huck was.   Huck was worried about the people in the circus, because he did not know the performers were trained.  The circus was fun and entertaining for everyone.  The duke and king provided rip-off entertainment to men.