Malvolio, whose name might be translated as ‘ill-will,’ is Olivia’s steward. Something of a puritan, he abhors disorder and drunkenness, along with bear-baiting and for that matter laughter: he does not smile.
He is disgusted that Olivia takes pleasure in the jokes of a professional jester. He also has a more than healthy self-regard; in fact, he is appallingly full of himself. His social aspirations are also somewhat unbounded: he dreams of acceding to the nobility, for which he believes himself well-suited. His fantasies about marrying Olivia are almost entirely non-erotic, focusing rather on the power, wealth and respectability he would thus gain. When Sir Toby pulls rank on him in the middle of the night, reminding him that he is nothing but a steward, his immediate reaction is to pull rank on Maria, who is even lower in the social hierarchy. He is therefore well-primed to interpret the letter in the garden the way he does, especially as it urges him to act towards others in the way that is his natural bent. Locked away in a dark room and left with no option but to beg the fool he has earlier humiliated for help, he is still unable to do so in a truly humble manner, and on discovering that he has been completely fooled, his only reaction is a promise of revenge.