One of the reasons writing mysteries (mysteries that are any good, anyway) is challenging is that you have to "get inside" so many heads. You have to think like a killer, suspect, and detective to realize what makes them tick and portray them successfully. You even have to think about what qualities make someone a target (the victim.)
For the next few blogs, I'm going to analyze these different groups of people that make up our mysteries . What motivates them? What do they worry about? What causes them to screw up?
Today, I'm focusing on the perpetrator: what prompts him to murder, how might he feel after committing the crime, how does he cover his tracks or make mistakes that lead to his capture?
What Makes the Murderer Kill?
Something in the killer's past leads him to kill: Many mysteries (especially thrillers and police procedurals) feature killers with disturbing backgrounds. Some were abused as children, some have had substance abuse problems and rocky relationships as adults. Psychological thrillers might delve into a killer's past to explain what led him to this point. In these cases, the murderer chooses--overtly or subconsciously--to perpetuate a cycle of violence that began in their past.
Desperation causes the killer to murder: Perhaps the perpetrator is desperate to keep a secret hidden (a local politician? A school teacher? Someone who needs to espouse an aura of trust for the town?) Many mysteries have killed off a blackmailer--either the murderer believed the blackmailer was about to expose him, or else the murderer was unwilling to continue paying out to shut the blackmailer up. Greed, a desperation for money, motivates many killers. Maybe the murderer is written into the victim's will and receive property or cash upon the victim's demise. In a desperate situations the perpetrator is trying to improve their lives by ridding themselves of a problem or by improving their lifestyle.
Thwarted love is another big motive. The murderer may kill for revenge on the person who stole their love from them. Or to eliminate the competition. Or to right what they see as an injustice (perhaps their nemesis treats their loved one poorly and doesn't appreciate what they have.) Here the murderer might be acting out of blind passion and the crime might not be as well thought-out if it occurred in the heat of the moment.
All right, so we've got the motivation for the first murder. But frequently there's at least a second in a mystery novel. Is the motive for the second murder the same as the first?
It can be. In the case of thrillers, you may have a deranged killer who is acting out as a result of their troubled past. Or perhaps there's a new blackmailer that has taken the place of the first one (a friend or confidant of the first victim.)
Sometimes there may be a different motive for the second murder. Frequently the second victim realized the identity of the murderer. Or else had some vital information or clue that would likely lead investigators to the killer. These victims are "hushed up." It's another example of a crime of desperation.
Now that we've thought a little about what motivates a murderer to kill, let's focus on:
How the Killer Might Feel About the Crime
Desperate: In many Agatha Christie books, the perpetrator makes more mistakes as his increasing distress that he might be discovered pushes him to continue the crime spree. In his rush to eliminate people who know he's guilty, he may leave more clues to his identity.
Cocky: In some cases, the murderer may feel as if they've gotten away with murder. As time goes on, he may feel invincible as no one yet discovers his identity. He has control over life and death--he chooses who lives, who dies. This power trip may encourage him to kill again. And his cockiness may cause him to make mistakes. Maybe he drops clues in conversation---he's just barely holding back on his need to brag about his accomplishments. He may reveal information that only the killer would know. Perhaps he drops physical clues that lead to his discovery.
Guilty: Maybe he's appalled at his actions. He may flush when questioned, be unable to look anyone in the eye, withdraw from the community. Or, as a red herring for the reader, the author could wonder whether his behavior indicates that he's covering up for someone else--someone close to him. Or the author might broach the fact that perhaps he has some information that could be vital to the case.
Covering Up the Crime:
We've already discussed one way the killer might try to cover his tracks---by killing again. Other means may be pressuring someone to provide a fictitious alibi (which can eventually be revealed), manipulating evidence (perhaps even attempting to return to the scene of the crime to remove an item that might point their way), and leave evidence or tell investigators information that leads them to another suspect. Sometimes in covering up the crime, the perpetrator may inadvertently bring attention to themselves: because the murderer is never smarter than the detective.