Is Slither a horror film? No. And I think that's why it wasn't as popular as it should've been. This is firmly in the comedy zone, even moreso than its spiritual ancestor Night of the Creeps. It doesn't attempt to be scary or atmospheric. But it rises above the Scary Movie 4 School of Suckage by taking its characters seriously and raising the stakes in a natural, believable way. You care about these people, even the reprehensible ones, and you want to know what happens to them. And by "you," I mean "me," because I have no idea if you have the same sensibilities as I do.
Which explains the other reason why this film was a big ol' flop: the target audience must be really really low. Sure, horror fans are a vocal bunch, and when we want to, we go to the movies in droves. But horror-comedy fans come from a slightly narrower bracket, because this implies that you're a horror fan, but that you don't absolutely HAVE to have shocks or suspense to have a good time. It's like finding a rollercoaster fan who likes riding them for the heights. I am a huge supporter of horror-comedies, but I understand why so many people aren't. It's a specific style, and you gotta be on the right wavelength. And by "you," I mean "you" this time. Go see Slither.
No reason to use this picture. I just enjoy the undertones.