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.