I always thought it was odd how so many horror movies end with the monster/killer beaten and the protagonist(s) escaping. To me, a happy ending usually makes a horror movie less scary. This ending tells the audience "It's okay, it can be beaten. Everything turned out okay after all." and it makes the monster less frightening to the audience. An ending where the monster/killer wins tends to be scarier, because it helps to drive home one of the most essential aspects of horror: The sense of vulnerability, a monster that ultimately wins makes the audience feel vulnerable and consider their own mortality and weakness.

They'll look at the dead hero and triumphant monster and see themselves in the same situation "If the hero can't kill that thing, what chance would I have?" they'll think to themselves. A good example of this is the fiction of H.P. Lovecraft. His stories often contain beings so monstrous that it's impossible for a human mind to even comprehend them, much less fight against them. This makes the threat of creatures like Yog-Sothoth all the more credible. A nigh unstoppable abomination should be just that, any attempt to shoehorn a happy ending would just damage the stories credibility.