Allow me to step in here and throw my 2 cents around...
Historically, there are varied accounts of massive floods throughout the world. However, they are not all at the same time. A flood could simply be caused by a tsunami, an earthquake, heavy rainfall, a natural levy breaking, volcanic explosion, etc. We still have those to this day.
When the writers of the Bible speak of "the world' we can only safely assume they are referring to "their world as they knew it," or the area of Northern Africa, Southern Europe and Western Asia. They knew nothing of Australia, the Americas, Antarctica, Pacific islands, Greenland, etc. If you take it literally as "the whole world," then you have to ask yourself how was it possible for people from say Peru to travel to the Middle East, in a time when intercontinental travel was extremely limited. You also have to keep in mind that the Bible is written from a Hebrew perspective, and that other cultures throughout the world disagree with their accounts. Some of those civilizations predate any Hebrew writings.
To answer the question of mass genocide, you have to step out of a modern mindset. Back then, it was survival of the fittest. You did not leave anyone alive who could grow up and seek revenge. If you want to take the 10 Commandment route about not killing, or loving your neighbor, you have to remember that the Big 10 only applied to the Hebrews. Everyone else was a Gentile, and not subject to Hebrew law.
God never said he made humans "perfect." He saw the creation and said it was "good." However, the implied principle is that humans were given free will, turned to "the dark side" (insert my best Darth Vadar voice here) and it is for that reason that humans were killed by God by way of disasters.
Ironically, there is an older Egyptian story about the flooding of the Nile. A man build a boat/ark, put his family, livestock and possessions on it, and when the Nile flooded, he rode it down stream. We know for a fact that stories were passed down and grew through cultural exchanges. The Romans were notorious for "borrowing" from their conquered.
I am not saying that the Noah story did not happen, but I highly doubt it was a world wide event. There's too much historical counter-evidence. I am firmly comfortable accepting the idea of "the world as they knew it."