There are a lot of great scenes in 'Salem's Lot. (Danny Glick clinging to the window, anyone?) The end isn't really one of those scenes, though. King probably should have ended it with the Barlow staking scene—you know, when Ben Mears tells all the vampires clustered around: "I'll be back… for all of you" (14.1259).
That's a dramatic moment: it's got punch and pizzazz, gushing blood and clustering nightmares. Or he could have ended it when Ben comes back to double-check and finds Barlow's teeth still sitting in the coffin—and they twist in his grip. That's icky and kind of funny. It works.
But you don't write a 650-page book by ruthlessly editing yourself, and King takes it further. Ben and Mark go away to Mexico, where they do this and that. Then they come back and set a fire to burn out the vampires' hiding places, so that they can hunt them down during the day.
The fire is an echo of the earlier giant fire in Jerusalem's Lot, so it's meant to cleanse, in part by returning the Lot to an earlier time. "[T]hey say fire purifies," Ben announces. "Purification should count for something, don't you think?" (Epilogue.42). It's like he's trying to convince the reader that this is a fitting end, like it makes sense to end with setting a fire rather than with a bloody staking or twisting teeth.
You might find this ending anticlimactic. What we've got here is Mark and Ben planning to burn out and stake vampires, but we don't get to see that happen.
It doesn't really end there, though. King wrote a short-story sequel to the novel in 1977 called "One for the Road" (collected in the short story volume Night Shift), in which it's revealed that the vampires weren't burnt out at all but are still around feasting on any unlucky tourist who happens to pass through.
Well, that's a scary idea. We'll go with it.