Generally speaking, most stories will have a main protagonist: a likable, 'good' character. And an antagonist: a 'bad guy', enemy and trigger of a 'crisis'. The characters definitely doesn't always appear as a 'hero' or villan'; it's just the easiest way to phrase the character-types. Effective story structures have to have to contain a 'crisis' of some sort, so having a character opposing your main narrator forms a well-rounded plot. For example, in a thriller the detective would be the protagonist and the serial killer his opposite, the antagonist.
Making 'villans' seem realistic by giving them some humanity is important. For example, serial killer events are quite rare so a local detective with little experience isn't going to be dealing with the brunt of the case. Just like a rivaling love interest could be jealous and confrontational, rather than passive. Stories work better with challenges. That's what the 'villainous' character-type is there to do.
A sense of realism and humanity for these charcter's is achieved through, for example, showing strong emotion or a hard-hitting back-story. Making the reader want to sympathise with the villain is an effective way of creating a more 'human' character. Upon the final breakdown on the 'villian' (or in terms of story structure, the 'crisis'), the serial killer becomes careless and is caught, the rival love interest physically assaults the couple and the kidnapped child escapes. The reader needs to understand the 'bad character's' motivations: whether that be childhood trauma, fear of rejection or addiction. A vulnerable aspect of the villain makes them a more realistic, well-rounded character.
On the the hand, your good, 'hero' character should also be well-developed with contrasting character traits to the villain. The hero should also have a hard-hitting history that makes them seem vulnerable and evoke sympathy. But, the good character has learnt to deal with their experiences in a positive way, rather than provoking a sense of rejection or revenge. Which the 'bad' character has developed.
Any more character-types I should cover?