One take on it:
Thrillers work the movie, building up to a major finding or turn of events in the plot with about 15-20 minutes remaining. Then the next 10-15 minutes play off of that, usually resulting in some sort of dynamic finish involving the death of the character in which the major finding was about.
Horrors usually feature a being (human or non-human) responsible for repeated torment (i.e. death) of an individual or numerous individuals. The film will usually culmulate in the death or destruction of that being. Exceptions to this last statement are films where a sequel is evident (like Freddy vs. Jason, even though they did "destroy" each other).
To sum up, the "major finding" for Thrillers is usually an emergence of an event late into the picture which takes the "ride" to a whole new level. Good example of this is No Way Out.
"Major finding" for Horrors is usually an emergence of some method or process to stop the tormentor. Unfortunately, the realism is usually thrown out the window because it's usually the stupid kids in the movie that figure out how to do it. Most horror movies usually end in disappointing fashion but there are some very good ones. Example of this is Nightmare on Elm Street.
Contradiction to this explanation is Texas Chainsaw Massacre.
Listen, there are many ways to answer your question, I just feel this was one of them.