1) A good working definition of horror could well be: that which remains after all attempts to "draw the line" have been exhausted.
2) When a person's sense of justice is satisfied, this is in indirect proportion to the amount of horror involved.
3) The universe/god does not draw the line on that catastrophic and hypothetical asteroid following a direct path toward earth, hellbent on our removal from existense.
4) My mother did not draw the line on the overwhelming volume of mental suffering her suicide would cause her family.
5) Someone else's tragedy is another's movie of the week, to varying degrees of intensity, depending on your cable network of choice.
--How the reader or watcher's imagination works with the medium presented, is the key factor to being truly horrified.. In other words, horror is almost entirely personal. There are aspects we share, if you give credence to jungian archetypes, but ultimately we color our own world with our very uniquely tuned senses and memories of past events, and horror films and novels will always be sifted through this personal mind set.As for the "boo" factor, it can be cliche, and it is in most horror movies, but certainly the most stand-out horror films use it in new ways-- something carefully planned (like Regan McNiel doing the spider walk down the stairs, truly creepy moment in The Exorcist)-- with well-prepared backstory, atmosphere and score. Something truly shocking rather than this overused one: "Oh it was just the cat!" We briefly recover, then, the killer strikes.