Horror is way too subjective to define... every individual has their own definition of what constitutes horror (just as you've demonstrated in your post above Ms. Jackson
).
A lot of the films I have and will list as horror will undoubtedly be questionable to others (just like Pedro's opinion of Aliens and They Live not being horror a few posts back), although I find genres by definition (especially where cinema is concerned) pretty hard to define; mainly because there is so much overlap, and room for interpretation.
Taking all the above into account, I don't really care for categorising by genre at all.