I did not say that I consider it the greatest sin. The Fall does have claim to the greatest tragedy in the spiritual sense - separation from God. "Greatest tragedy" here may be hyperbolic. I was really trying to draw attention to the fact that pretty much every human was killed. It is a disaster worse than pretty much every post-apocalyptic movie and TV show.
Concerning the angels/demi-humans, Genesis 6:5 (NIV): The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time.
To me, while the angels may have been responsible for great evil, humanity had chosen a path against God.
Your point about the rainbow is good. It is a beautiful symbol for such a grim sort of promise though (it is a positive promise, but a promise not to destroy by flood does feel a bit grim).
Many of the stories in the Bible are difficult to share directly with young kids, so they get lightened a bit. It is understandable. However, I find the story of Noah to have the greatest contrast between the story in the Bible and the story as we present it to kids. That is why I wrote this poem.
For example:
RE: Postdiluvian Disorder