CIRQUE DU FREAK by Darren Shan is YA, not middle grade. He has a demon series that is very scary but very YA -- gruesome.
Mary Downing Hahn writes middle grade horror -- Wait Till Helen Comes: A Ghost Story and the others are definitely middle grade. Very creepy, atmospheric, and middle grade readers find them very scary. They aren't at all like the campy style of Goosebumps.
Skeleton Creek by Patrick Carman is probably pushing toward YA...it's an interesting format (told half through journal entries in the book, half through video entries online. It's very scary but I would consider it "middle grade-esque" -- there is nothing inherently "teen" about it except that it's scary.
Vivian Vande Velda has done some middle grade horror (though she often gets bumped up to teen because of the dead people in the books, but some of them have a very middle grade vibe --there is one with a civil war era ghost and one with a hanged guy...very scary, a little wry but not at all campy like Goosebumps. The titles reflect the wry voice, but they are not comedies.
Skeleton Man by Joseph Bruchac is supposed to be very scary -- I haven't read it. It's definitely middle grade.
The Hanging Hill by Chris Grabenstein (and The Crossroads, the first book in the series) are PERFECT middle grade horror. Very scary. Not funny. But tempered for kids. If you really want to understand how to do middle grade horror -- Grabenstein and Vande Velde should be required reading.
The Suddenly Supernatural Series by Elizabeth Cody Kimmel is very much a middle grade girls ghost/horor series. Creepy more than gross (so think more atmospheric and less goosebumps.)
Now, I don't know if the book is out yet, but I have a copy of MALICE by Chris Wooding and it's middle grade that is super, super scary. It's a combination of straight narrative with comic book entries (which makes perfect sense with the storyline). Really wonderful horror story.
In the boxloads of books I get for review, I'm seeing an ever growing pile of middle grade horror. So, it's out there -- to really understand it though, read Mary Downing Hahn, Vivian Vande Velde, and Chris Grabenstein -- you'll be glad you did. It doesn't have to be silly, campy or light.