Symbolism, Imagery, Allegory
Much like the vampires, the Marsten House is a symbol of evil. Hubie Marsten was a horrible man who killed many, many people, and the novel suggests that that evil lives on after him, that all of his unspeakable acts left a "dry charge" (2.262).
Marsten's evil brought Barlow to the town and gave him a place to stay. The Marsten murder-suicide has made the Marsten House "unhallowed ground," according to Callahan (14.236); the house sits above the town "like a ruined king" (15.14). It's a sign of the town's evil and corruption, which it both reflects and kind of caused.
In short—bad Marsten House! Bad, bad Marsten House! Shame on you!
Nobody's All Bad
But, is the Marsten House all bad? It's certainly an evil place, but it's just as certainly Ben's evil place—the place in which he had a fright as a child, and which he sees as a center of everything bad. For just that reason, it's also a place of controlling evil, or of making sense of badness.
Ben came back to the house to "Confront… my own terrors and evils, maybe… Or maybe just tapping into the atmosphere of the place to write a book scary enough to make me a million dollars. But no matter what, I felt that I was in control of the situation" (5.104). The Marsten House as a symbol is usable; it summarizes all the evil in the town and in Ben's life so he can get a handle on it.
Ben goes on to say that things are spiraling out of control: the house is occupied, Ralphie Glick is dead, and the house is fighting him. But the slightly hysterical rant seems unconvincing.
And no wonder. Because, while Ben's book about the Marsten House may be out of control, the actual book about the Marsten House, the 'Salem's Lot you're actually reading, isn't out of control. It's doing exactly what Ben wants: it puts the Marsten House, that font of evil, in a story with a beginning, middle, and end, where terrors are (safely) confrontable.