Cruelty of men is expressed in many ways through different characters. In "Twelfth Night", we find that many characters do possess a sort of cruelty in them - and this cruelty changes from something seething within them into something which they inflict tremendously on others, in particular, Malvolio. Despite Twelfth Night being a comedy, dark humour does prevail in this play, and ends in hideous cruelty shown towards Malvolio. .
There is a reference of cruelty within the character of Olivia. Olivia is described by Orsino as "fair cruelty" and "sovereign cruelty". Orsino must have felt mistreated and unjustified in receiving unrequited love from Olivia, and thus labelled her as a cruelty. It is also evident that Olivia seems to hold herself in high regard and thinks Orsino unworthy of her love.
Orsino himself, is infatuated, and it is his deep love for Olivia that makes him capable of committing violent deeds as well. When he realizes finally that Olivia will not marry him, he is willing to inflict all cruelty on his most loyal servant Cesario and "sacrifice the lamb that [he] loves".
Sir Toby, in asking Sir Andrew to stay and through that eventually inflicting emotional hurt on him, is cruel. The only reason he wanted Sir Andrew to stay was so that he could enjoy the pleasures and revelry that Sir Andrew's money would allow him to indulge in. Knowing Sir Andrew's weakness and gullibility, and that he is "excellently ignorant", Sir Toby takes advantage of that and convinces him to stay under the pretense of raising his hope of winning the love of Olivia. .
Sir Toby also finds great entertainment in forcing Sir Andrew and Viola to fight. He hopes through this to see both parties awfully scared like "cockatrices", and this somehow provides a source of amusement for him.
Another instance of the potential of dreadful cruelty within Sir Toby is the "gulling" of Malvolio. Together with Maria, Sir Andrew and Fabian, this group of friends come together with a plan which exploits Malvolio's blindness and self-infatuation, to provide a "joke" for them to laugh at, by embarrassing him and causing him to lose all dignity.