In a crime story, there's always a protagonist and Antagonist. The protagonist is the person who is often affected by the crime or the event that's happening in your book. The person in question will probably suffer some kind of loss, which will motivate them to solve the crime or the issue that's taking place. It is common for a protagonist to be emotionally invested in what's going on and be affected in some way by the crime. Even if the protagonist doesn't know the victim, perhaps there is some kind of emotional connection that will rattle him or her.
The antagonist, on the other hand, may not be well known to the reader. But that doesn't mean that you, as a writer, should completely neglect a character description of the antagonist. Of course, it is scary and it is fearful to not understand what has happened in a crime. You don't know who the villain is and it could be anyone in your social circle. But as the author, you need to understand what could drive someone to commit a crime. It's important for you as the author to understand your antagonist as well as you do your protagonist.
Readers need to understand
While you may spend a lot of time building up your protagonist to be a relatable character, it's important that you as an author understand why your antagonist or your villain does the things that he or she does.
If you're dealing with a killer, why is he or she killing the victims? Does he or she have childhood issues or abandonment issues that need to be addressed? Is there a reason why you are targeting this particular villain for your book?
Readers may be on the side of the protagonist in solving a crime, but you are developing a sense of fear by developing your antagonist.
If your antagonist is just the neighbor, who is being accused of killing a local girl, readers can relate because they have a neighbor. All of a sudden, your antagonist just became really scary and frightening. By giving them an identity, you are making the character more real to your readers.
Creating an understanding
Even though you are developing your antagonist to have feelings, emotions, and even a motive for the things that he or she does, it is also interesting to look at this person as someone who could be understood.
Of course, many readers do not want to feel any sense of empathy or sympathy for the villain. But if you look at the television show "Dexter," the writers managed to make people fall in love with a man who is a Serial Killer. While he says he only kills the people who have done something wrong, fans of the show are still cheering for a serial killer. It's reverse psychology and it could help you in your novel if you try to do the same thing. Sometimes, it's best to create characters who are trying to be understood as your readers will be more inclined to give them a chance.
Do you give your villain a motive in your novels every time you write?