I think many of the monsters are manifestations of not simply guilt, but psychological trauma, nurosies, obsessions, fears, guilt, psychosis, and other psychobabble stuff. yes I took psychology... wheee...
I believe that whatever demon/god/great old one, is tied to silent hill likely creates the conditions with the monsters only manifest to, and for, those who have problems.
This was illustrated by one of the few characters not affected by The Mists, the head of the Silent Hill Cult, (he appeared in SH 2) he didn't beleve any of the cult's bull, he was more into studying phenomina and taking financial advantage of the cult's devotion. His sanity kept his perception in reality, while he appared in the "mist world" as a normal human and had a conversation with the game's protaginest.
She percieved the mad cultists as the monsters they'd become, while he, thanks to his sanity, was unaffected, and saw the cultists as human beings (though mad and dangerious humans)
Nearest I can make out there are three sides of Silent Hill, all are mirrors of each other
1. the real world, a relitively small town near a lake, perfectly normal save the aura of creepyness, the real world is mostly safe.
2. the mist world, the first step is more along the lines of "Unsanity" instead of outright madness, this world is where many who are "cracked" in silent hill wind up dissapearing to. Shrouded in cold mists that seem to give shape to fears and psychosies this world is dangerious and inhabited by monsters that are made from twisted flesh.
3. the hell world, in contrast to the cold madness of the world above this world made of metal, rust, flesh, and fire. often with a massive fire somewhere deep below with warm winds of ash flowing. the Hell World is where most of the boss monsters and creatures created by serious problems and traumatic events arise. details are sketchy but I don't think that it's a coinicindance that creatures like "Great Worm", "Pyrimid Head", and "Suture" are encountered here.