Watch any reasonably competent actor playing the part and you will know. Get hold of a copy of Act 2 Scene 5, with Malvolio played by Richard Briers, or Nigel Hawthorne, or Alex Guinness, or Michael Maloney or someone like that. Watch it.Malvolio is what in Shakespeare's day they called a "humorous character", a person dominated by a personality quirk, generally an unpleasant one. Malvolio is vain and stuck-up. He dreams of a romance with Olivia, his employer and social superior. He sneers at all the other characters in Olivia's household. The audience begins to think he is asking for a swift kick in the pants, and Maria, Toby et al are happy to deliver.