Horror-Comedy is a curious animal, much like the curious raccoon ransacking my garbage in that I don't like it. There are exceptions, but generally comedy and horror work best when they build up unimpeded. It's easier to laugh out loud when you're already grinning because ha ha! Chris Tucker's voice is so crazy! And you're more likely to feel frightened when tension or foreboding has already been gradually established, like if the exposition is Chris Tucker ominously screaming Edgar Allen Poe's "The Raven." Severance, Slither, and Hatchet all try to blend what I see as contradictory genres, and they do so with descending degrees of success.
Severance is a modest British film I'd never heard of until I saw it, and it's probably the funniest horror film I've ever seen, insofar as that's possible. Slither is sort of the opposite side of the horror-comedy coin; where Severance is dry British wit with a semi-realistic horror context, Slither is an over-the-top parody of B-movie horror.
Severance stumbles a bit in occasionally locating its horror only moments from its comedy. For some people that might work, but I've never felt comfortable when the intended audience reaction is "ha ha ha OH SHIT AUGH" (with a possible exception for the Coens, but you pretty much have to exclude them from any generaliztion you make anyway). When it wants to be, though, this film is by turns unsettling and very funny. It also benefits from the surprisingly good acting of Laura Harris, whom I only know from 24, but who has apparently been in a million things since she was twelve, including gems like M.A.N.T.I.S. and Sliders.
Slither is basically an amalgamation of Shivers by David Cronenberg and Night of the Creeps by, uh... I don't know, somebody. Slither is less serious than Severance throughout, and when the film falls short it's because it relies too heavily on redneck jokery (as noted by the Onion A.V. Club). But it's still likable enough to work, with enough characterization beneath the parody for us to stay interested. It's cinematically coherent. It's not Hatchet.
Hatchet is... Well, I'm not sure what it is. I think it's trying to be a genre parody like Slither, but everybody in it is so criminally unlikable that I really don't give a shit what its intentions were. It's like a Wayans horror. It's a series of jokes I don't find remotely funny, revolving around sophomoric stereotypes, about people I absolutely can't stand. Also there's a monster in there somewhere, and it's stupid.
The senselessness of these boring idiots must have been intended as a joke, but here's the problem: people in a movie have to behave remotely sensibly, or if not, they at least have to be stupid in a way that's funny. This film, on the other hand, is just an exercize in random, unordered events and detestable characters who do 80 minutes of unfunny shtick while dying. In the process they behave like mental patients. Mental patients who can't act or tell jokes.