How did Banquo facilitate triumph over evil and how did he prevent it in Macbeth?

Banquo, according to the witches, is "Lesser than Macbeth
and greater."  "Not so happy, yet much happier."  In this case alone, Banquo is the
greater man.  He is more in tune with integrity, honesty, honor, and doing what is right
just because it is right.  The witches know that Macbeth will be swayed to take matters
into his own hands instead of allowing nature to take its course and bring him to the
prophecy's truth of its own accord and in its own time...meaning, he doesn't want to
wait for the natural course of events.  He is impatient and greedy, and of course,
spurred on by his equally impatient and greedy wife.  It also bears mentioning that
Banquo's character is clearly more toward light than darkness since he did not wholly
swallow the prophecy of the witches--he is skeptical and laughs it off, whereas Macbeth
begs them to say more, and he clings to their words as if it were the water he needed
after a long desert crossing.

Banquo, however, would not
bend.  He is not as high ranking (lesser than Macbeth) but he will not stoop to crime
and injustice to better himself (greater than Macbeth).  He is true to his word and true
to his loyalties (Happier than Macbeth), yet he will not ever be King (less happy than
Macbeth).  Banquo's happiness lies in the fact that his is an honest life and that his
sons will one day be King of Scotland although he will never know the feel of the throne
and the crown. 

Banquo does suspect that Macbeth has acted
"most foully for it" once the King is dead and Macbeth is to be crowned.  His
suspicions, along with the prophecy of the witches, put him in the greatest danger since
Macbeth is on the warpath in an attempt to protect his own future.  Banquo's refusal to
attend the banquet (I believe he does not intend to eat with Macbeth as MacDuff does not
attend either the coronation or the banquet)--or at least not to be there on time-- due
to a short trip with his son, Fleance, sets him up for the trap set by the hired
murderers.  His ability to get Fleance to safety by yelling, "Fly, Fleance, Fly!" is one
way he thwarts evil from taking over.  By keeping his son safe, the prophecy of the
witches that Banquo's sons will one day sit on the throne with a long lineage, is also
safe.  This news is detrimental to Macbeth's security as King and dementor of
Scotland.