I remember The Bookish Babysitter.
I also remembered that there were other episodes with dark endings but The Pinball Wizard was the only one I could distinctly recall...*looks through the summaries again*...I guess the Thirteenth Floor, although that one is a little different. That's one of the more Twilight Zone-esque stories.
I do not remember the season 5 episode Dead Man's Float (or anything from that season) but I'm seeing it on lists of people's favorite episodes and it has this in it
That's a pretty gruesome monster for that show.