Mark Petrie is a kid, and 'Salem's Lot thinks kids are awesome. So Mark Petrie is awesome.
Okay, so not all kids in the novel are awesome. Richie Boddin, the bully Mark beats up because Mark is awesome, is not awesome—he's a clumsy jerk. Danny and Ralphie Glick are nice enough, but not awesome, which is why the vampire gets them.
But Mark isn't any kid. He's a kid who sums up the kidness of kids. He does this in large part by being the symbolic younger self of Ben Mears (see our "Character Analysis" for Ben Mears). Ben, who is writing a book about 'Salem's Lot, is a stand-in for Stephen King, the novel's author, which means that Mark is a stand-in for Stephen King's vision of his own childhood. Mark's an idealized, awesome vision of Stephen King as a child
Such idealized, awesome children appear throughout King's work. As one critic says about It: "There's constant talk about how kids are superior to adults in every way" (source.) 'Salem's Lot isn't quite as insistent, but still, the awesomeness of kids comes up quite a bit. After Matt vanquishes Danny, for example, we get this:
The night before, Matt Burke had faced such a dark thing and had been stricken by a heart seizure brought on by fright; tonight Mark Petrie had faced one, and ten minutes later lay in the lap of sleep... Such is the difference between men and boys. (10.436)
Along the same lines, Susan is "ashamed" of doubting the existence of vampires when faced with Mark's simple, honest belief (12.27-29).
Mark's superiority to adults is, as this suggests, in the ease with which he believes in the supernatural. That belief is tied to his consumption of monster magazines and monster movies. Children (or at least ideal children) are awesome because they are connected to the supernatural, and that connection is also a connection to, you know, pulp magazines about monsters.
Matt calls Mark a "scholar" of vampires and monsters (14.123), and the boy's taste for gory excitement and suspense thrills saves him repeatedly. He drives off Danny Glick with a plastic cross from his monster set.
Even more spectacularly, his reading about Harry Houdini allows him to escape Straker's ropes. The trance he puts himself into as he escapes shows us that he's pretty much become one with the adventure narratives he reads: "The extreme level of his concentration had put him in partial control of his own sympathetic nervous system" (13.146). It's as if Mark has gone from simply a reader of suspense to being both inside and outside the book, an author of himself. He's turned into Ben, who he'll soon meet—or, if you prefer, he's turned into Stephen King.
Mark=childhood=pulp suspense enthusiast=pulp suspense writer=Ben=Stephen King. It's a long equation, but the upshot is that kids are awesome. Especially when they grow up to write horror novels.