WMG: Trick 'r Treat

The Principal's candy is only poisonous if you eat a lot of it.
This occurred to me after I realized that if there was a mass death-by-poison in the neighborhood, sooner or later, it would be traced back to the source. So I came up with the theory that if kids follow the rules of Halloween, and only take one, as the sign on the bowl instructs, they'll be fine.

Sam is behind it all.

In all of the segments Sam is usually shown observing it. Which means that Sam might have orchestrated the events that have been happening in the movie and unleashed the supernatuarl creatures in the movie to punish those who don't respect the tradition of Halloween.

Rhonda was a real witch.
Or at least training to be one. That would explain her being nonplussed about the other kids' deaths or seeing Sam.

Sam was created by the last paleo-pagans.
When they realized that the Christians were going to wipe them out, they decided that they had to preserve something of themselves. So, they created Sam with witchcraft, who enforced the rules of Samhain, and who the Christians feared so much that they had no choice but to abide by the laws of a pagan holiday.
  • Apparently confirmed by Days of the Dead.

Sam saves far more lives than he takes.
Sam is the enforcer of the rules of Halloween, and he punishes anyone who breaks them, human or monster. We only see him punish humans, because humans don't know about him and thus break the rules, while the monsters are all fully aware of him and stay in line. However, in his absence the monsters would slaughter the entire town, with absolutely no fear of consequences.