Fabian helps the others plot revenge on Malvolio because the steward told Olivia about Fabian's participation in animal cruelty. Fabian is also the one who confesses when things come to a head.
When Sir Toby and Sir Andrew discuss humiliating Malvolio with Fabian, he responds positively. He says:
"I would exult, man. You know, he brought me out o' favor with my lady about a bear-baiting here."
This is his motivation for helping to embarrass Malvolio. At the end of the play, though, he decides to set things straight. He reads the letter from Malvolio to Olivia that shows Malvolio isn't insane. At Olivia's command, he also goes to free Malvolio.
When he returns, he confesses. He says:
"Most freely I confess, myself and Toby
Set this device against Malvolio here,
Upon some stubborn and uncourteous parts
We had conceived against him. Maria writ
The letter at Sir Toby’s great...
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importance."
He then says that everyone should just take it as a jest because everyone was equally offended. Fabian is a source of information for the primary characters in the play. He also helps move the plot along by getting involved in their schemes.
Fabian has a very minor role in
Shakespeare's Twelth Night, appearing only in Act II, Scene V; Act
III, Scene IV; and Act V, Scene I. He is essentially one of Shakespeare's
stock characters, a character type that appears over and over
again in Shakespeare's works. In particular, he can be classified as the stock
character identified as a servant and partially as the stock
character identified as a "clever servant" (Schwartz,
"Shakespeare's Plays: Comedy").
In Act II, Scene V, we learn that Fabian has fallen "out o'
favour with" Olivia because Malvolio tattled on him. Wanting
revenge, Fabian joins in on Maria's and Sir Toby's
plot to humiliate Malvolio by making him believe Olivia is in
love with him. It is in joining the plot that Fabian demonstrates he is a
clever servant. Plus, as both a prankster and a clever
servant, he gets involved in the duel between Andrew and Cesario in
Act III, Scene IV.