Iago, Fagin, Cruella de Vil, Gollum. The hideous creations of Evelyn Waugh, the moneyed monstrosities of F. Scott Fitzgerald (‘They were careless people, Tom and Daisy’). The White Witch, Tom Ripley, Emma Bovary (I know, I know – but I can’t bear the woman myself, yet she remains the central figure in one of my favourite novels).
Consider the great villains and unpleasant characters of literature. We are not told these characters are villainous or unpleasant, but we develop our sense of them through close observation of external details: their manners, their gestures, their actions, their desires (that Dalmatian-skin coat), in addition to dialogue and description. And these characters can become so beguiling, so compelling, that we might also come to like them.
In no more than one page, create a vignette that uses the third person and externally observed details (i.e., showing rather than telling) to introduce some vile yet compelling character.