Something about horror and comedy keeps pushing the two together. It's as if the same spot of the brain that screams at something could, under slightly different circumstances, laugh at it. Sometimes the scream and the laugh are simultaneous. This makes for a group of movies that are often called horror comedies, although it is more accurate to distinguish the horror movies with comic elements from the the comedies with horror elements.
I just rewatched a prime example of the latter, Arsenic and Old Lace (1944). It isn't particularly scary, but it is hilarious, and it uses horror themes to get its laughs--psychopathic serial killers, a spooky house with 13 bodies in the basement, and a man who looks like Boris Karloff (but is actually played by Raymond Massey). I first encountered this movie in high school, when I thought it was one of the funniest movies I'd ever seen. Later I read critics who thought it was creaky and overplayed, so my estimation of it went down. Now that I am older and care less what other critics say, I've gone back to my original estimation.
You won't find Arsenic and Old Lace (1944) on About.com's list of 25 Best Horror-Comedy Movies, but then this list doesn't make the distinction between comedies with horror elements (such as Arsenic and Old Lace, Ghostbusters, and Young Frankenstein) and horror movies with comedy elements (such as Re-Animator and Evil Dead II). The difference is whether the film primarily exists to present you with a deformed destructive being, thus tagging it as horror, or whether it primarily exists to make you laugh, thus tagging it as comedy.
Either way, there are far too many movies that don't do either task well: horror-generation or laughter-generation. I am thinking of Zombieland, which was not very funny or very scary, but managed to become a hit just the same. Then there was the more artistically successful but still problematic Severance, which was a little too funny, enough so that the humor undercut the otherwise well-done horror. Arsenic and Old Lace reminds me of what can happen when the filmmakers focus on one category with the other used sparingly.
Deformed and Destructive Beings: The Purpose of Horror Films