To create an edge-of-the-seat suspense, the author spends much time crafting the villain. A villain must be a worthy opponent. This has been covered in other articles on this blog, but it is worth reminding. The villain needs to be cunning, amoral or believes he has the right, and dangerous.
He must have valid motivation for his desired crime or continuing crime, such as: revenge, righting a wrong, protecting his possessions or loved ones, vindicating an action he believes went unpunished. He must have the opportunity to have committed the crime, and the appearance of innocence. This is why sometimes the protagonist becomes a suspect. He can be the charming insurance man or the kindly crossing guard. He can hid beneath his role in the story while the author provides only hints of information that can be put together like a puzzle to help tilt the protagonist in the right direction. The villain can be involved in the characters lives and appears innocent, but in some types of suspense, the villain can be known to the reader and unknown to the protagonist. Yet creating the villain follows the same procedure.
Killers can use many methods of committing murder. Select an appropriate method of perpetrating the crime (the modus operandi) that fits your character. Provide the villain with the kind of knowledge, know-how, strength and ability to be successful. Make his choice plausible. If he poisons someone, how would he know the poison will work? What career or opportunity would he have to steal or purchase what he needed? The old movie and play Arsenic and Old Lace is the story of two elderly women who poison lonely men with elderberry wine and bury them in the basement with their mentally-handicapped brother’s help.
Villains can be diabolical or as sweet as the two sisters in Arsenic and Old Lace, but make them real by providing them with realistic motivation, ability equal or greater than the protagonist, and a credible modus operandi to fit their physical and mental ability.