The story itself is not the truth, but a myth, happening in various cultures and mythologies:
https://en.wikipedia.org/wiki/List_of_flood_myths
Considering there is so many myths about the flood, and that all of them are either very similar, or almost identical in terms of the plot, one may assume that long ago, some big catastrophe may have happened, creating a myth that started spreading and passing on to other cultures. That’s how it got into the Bible.
As for the myth in the Bible, I think that God was not evil or demonic there. He created the world, and that world was being ruined with human greed and evil. I think God was disappointed and fed up with his beloved creation being ruined. He gave people will to choose between right and wrong, and they chose wrong. i tend to see it as “I’m so sick of this sh#t. We need a restart here” and decided to destroy life and create a new start. I think the Bible god is more of disappointed and fed up, rather than evil or violent, like, for example, Bel was in Sumerian myth. He simply wanted to wipe people out, without saving anyone, because they were too loud. (??? And no one point a finger at him…)
In short: it’s a myth. It may have been created from some devastating flood, but definitely not the world wide flood, and there was certainly not any actual Noah. I think the myth is interesting, it shows the nature of God. Like I said, I don’t see him as actually evil (he did decide to save the ones who deserved it, and made a promise to never do it again. I never saw the bible God as evil, he punished people a lot, but they kind of deserved it. He never was like: “Huh, I’m bored, how about killing people!”)