Check this book

Ever heard of the Book of Enoch? It's not widely accepted as Christian "canon" (although some Coptic Christians consider it to be inspired scripture) but it was mentioned in the Book of Jude (that's the next-to-last one in the Bible just before Revelation). I've read it a couple of times since first hearing about it several years ago. And it's quite an interesting read. Among other things according to the Book of Enoch, the Flood was sent by God to cleanse the Earth because fallen angels - who had originally been charged with taking care of creation - started having sex with human females. The result was a monstrous race of human/angel half-breeds and it got so bad, that the only people left who it could only be said were pure genetically human were Noah and his family: whole new meaning to Noah being "perfect in his generation". So God protected the one pure human remnant left and wiped out the genetic abominations that were befouling the Earth. So according to the Book of Enoch, that is what the great sin was that demanded the Flood.