What comes between people and reason in "To Kill a Mockingbird"?

The answer should be a simple one: emotion. However,
emotions can be manipulated by rhetoric, alcohol, the mob mentality, cultural mores and
norms.

If a normally rational person is provoked beyond all
reason, then that "sane" person will eventually become irrational in a fit of anger and
passion defend his or her space.

In the story, we see
Mayella Ewell caged like a slave to her father's whims. Yet, she finds a way to escape
the savage conditions under which she is kept even though it cost Mr. Tom Robinson his
life. Mayella acted irrationally due to conditions beyond her
control.

Humans resist and fear change.  The cultural
dynamic of the time was changing. Atticus Finch upheld the law regarding the defense of
Tom Robinson even though he realized that Tom's status as a colored man would limit the
outcome of the trial to go against his favor. Atticus maintained his reason in the face
of unreasonable circumstances and odds against him.

The mob
who came to lynch Tom Robinson was probably the most telling example of how emotions can
be manipulated by rhetoric and alcohol. The men in the mob all lost their reason due to
the  public outrage of Mayella's story and the addition of alcohol to fuel their rage.
In a moment of clarity, Scout Finch struck up a conversation with one person in the mob
whom she recognized as a normally rational, reasonable person. Her courage and bravery
turned the tide and disarmed the mob mentality.

Bob Ewell
is the one example of a person whose life is totally out of control and whose only
ability to be in control is by controlling his daughter, Mayella. He is well known to
the community as "white trash" and a trouble maker. His only standing in the community
is that he is "not black." Having the nerve to blame Tom Robinson for the attempted rape
of his daughter instead of admitting to the community that his neglect and drunkenness
caused her to reach out to anyone in the community, even a black man, shows the reader
just how unreasonable and irrational Bob Ewell has become.  His lie is found out when he
is asked to write his name; he is left handed.