The actual laughing (which we can observe from an audience during the goriest parts of bloody horror films) are NOT laughs of humour, but tension, hence the expression "laughing the tension away".
I don't laugh at someone else's misery, nor do most people. A Canadian professor gave some lectures about horror films last year, which were very interesting. Theory has it that humans have an inborn interest for seeing bloody, gory things. (Which explains the interest for horror literature/film/pictures) This might well be related with the fact that we used to lead a partly predatorial life, though I think we should discuss this in the "Humans runs to scavange???" thread.