What does Malvolio's behavior at end of the play show about him?

At the end of the play, Malvolio speaks with composure, and in verse, for the first time. He is not bewildered, as he had been throughout Act IV, but rather he seeks vengeance for the wrongs that he believes Olivia has done to him. Olivia, however, has done nothing to her steward; she defuses the situation very carefully, promising him justice, and speaking very politely, so as not to inflame him any further.

But it is Feste who explains the situation, and stirs up Malvolio's anger once again. Just as it appears that Malvolio is satisfied with Olivia's pronouncement, Feste mocks what Malvolio had said to "Sir Topaz," and Malvolio's disdainful comments to him after he catches Feste and Sir Toby making a stir at night. Feste airs his resentment of Malvolio before the whole party, and turns it into pointed criticism of Malvolio; Malvolio, in return, is greatly angered, and swears that he will get revenge on Feste and his cohorts for what has been done to him. There is no real closure in Malvolio's situation; he ends the play just as high-strung as he had begun it, and rather than gathering any truths about himself from Feste's statements, places the blame on them and feels pumped up by self-righteousness once again.

At the end of the play, Malvolio still has no knowledge of his failings; although the pranks played on him were meant to punish him for his pride and vanity, he has still not seen the error of his ways, or tried to change himself. Feste's statement about how his enemies "tell me plainly I am an ass, so that by my foes, sir, I profit in the knowledge of myself," could be taken as a justification for the whole attempt to bring Malvolio to penance (l. 16). If this declaration was indeed Feste's motivation in tormenting Malvolio, then in retrospect, his actions in Act IV were not as cruel as they seemed to be.