By its simplest definition, a thriller is a story in which things go very, very wrong. Whether the threat comes from an erupting volcano, escaped nanobots, a political assassin, or bomb-toting terrorists, the pattern is the same. As the story progresses, the situation deteriorates, until it seems as though there is absolutely no way the story can end well.
A mystery is often described as a whodunnit. A thriller is more of a "how are they going to get out of it." Suspense, tension, and excitement are the hallmarks of a good thriller. If a thriller doesn't thrill, it's not doing its job.
A thriller must finish with "a final confrontation between the hero and the villain, both verbal, and physical," according to New York Times bestselling thriller author Gayle Lynds.
Such a confrontation necessarily involves violence. To what degree, however, is up to the author. Savvy authors know that frequently, tension comes not from the actual act, but from the threat of the act. What the reader imagines will happen is often worse than what the author has planned.
Readers have limits when it comes to the amount of violence they'll tolerate in fiction. Writers have limits too. This is because in order to write a scene, a writer not only has to put themselves inside the action, they have to go deep inside their characters' heads -- both the good guys, and the villains. So deep, that in a sense, the writer actually becomes the character. The writer has to know what the characters are thinking and feeling, what they're afraid of, and what they want, no matter how dark or aberrant their motivations. If the writer can't -- or won't -- go that far, then the reader can't follow, and the scene falls flat.
I don't shy away from violence in my science thrillers, and I don't hold back from killing my characters when the story requires it. But the violence in my books is what I consider "natural" -- man against nature (or perhaps more accurately, nature against man), rather than man against man. I don't have a problem with Crichton's dinosaurs eating people any more than I do watching a Discovery Channel program in which a lion takes down a gazelle. But imagining what it's like to stick a knife into someone's belly and pull up on it to gut them -- I don't want to go there. Just writing that sentence leaves me feeling squeamish. I don't want to imagine what it feels like -- on either side of the knife.
Which is why I won't write a disaster thriller. Whether it's an asteroid striking the earth, or a cruise ship turning upside down, the scale and scope of a disaster thriller require a very large number of bodies. A great many people die, randomly and horrifically, in fires, explosions, crushed by falling buildings, when the ground opens up, when the plane they're a passenger in falls out of the sky. In a disaster thriller, life and death depend less on intelligence and ingenuity and more on pluck and luck.
I don't want to write about random horror and death. There's too much of that in real life. An earthquake that generates a tsunami that devastates a coastal city and damages the cooling systems at several nuclear power reactors threatening multiple nuclear meltdowns reads like the plot for a disaster thriller. Tragically, that's a disaster none of us needs to imagine.