The first DM I ever had was my best friend's dad when I was in the 4th grade - his whole family played D&D as a family-time game. No strangeness there, except perhaps the most June-and-Ward-Cleaver D&D game I can ever remember playing.
But my
second DM, through 6th-8th grades, was a really scary guy. He was in college, but he only played with high school and jr. high school students. He always had his fingernails painted black with strange pictures or symbols painted in gold on top. He made us memorize our character sheets, so we could play in faint candlelight - just good enough to see our dice rolls. From the time we entered his game room, til we left, EVERYTHING we said was "in character". And if your character died, you had to leave, and essentially beg him to spend the time on helping you make a new character - always first level, of course. :\
In the town I lived in, every Halloween several people's housepets would go missing - to turn up later decapitated. And one year, it happened to a small child. Whenever the subject would come up, our DM would always have a knowing little smile on his face, but wouldn't say anything.
When I read the Chick Tract on D&D, I kinda thought he must have met my DM. I'd have probably been ready to think the whole game was some sort of gateway to Satanism if it hadn't been for my earlier experiences with my friend's family.