One thing I remember clearly from the novel was Pennywise having "silver dollar eyes". What exactly that means its up to interpretation. Perhaps they looked like shiny coins, or perhaps its a reference to coins laid over the eyes of the dead. Maybe the irises or entire eye was like liquid silver, or perhaps the glinted with a cold metallic light.
The bad endings reflect Bill's view on the world.
The endings are considered bad because they are sad. At some point in the movie (I can't find the script, or subtitles as of this date) Bill complains to the dislikers that his endings, although bad, are realistic.
Life isn't always a fairy tale, and things don't always end happily.
This reflects Bill's ...