Undoubtfully the all-loving G-d was a bit harsh on punishing the whole generation of Noah (probably millions?) by drowning them (and all the animals - what did they do?) in boiling water (He could simply give them a stroke). But what were they punished for? Here's some background, please let me know if I contradict any Midrashic interpretations:
G-d creates Adam and allegedly commands him the 7 Noahide laws with nothing in written (for those who think only Noah was commanded, this is even a better question).
Even when commanded, Adam is not explained the reward for his compliance or the punishment for transgressing them, not mentioning "the final solution" for the whole of humanity.
Strangely, we see that Adam isn't commanded to pass those commandments and this knowledge on to his descendants or to rebuke the wrongdoers. Neither he's commanded to educate his kids accordingly. Surely he wanders around telling his story, but the only punishment he tells about is downgrading to the Earthy life.
G-d gets, probably, seriously hurt by Adam's sin and decides to completely hide Himself from humanity, He doesn't speak with people and doesn't send reliable messengers.
Meanwhile, 9 generations of humans are born into total bliss and anarchy, setting their own rules for social conduct and eventually forgetting who G-d is. That angers Him but, instead of intervening, He patiently waits for almost 16 centuries and then suddenly, one day, He reveals to only one man, describes His plan and commands to prepare himself for his personal salvation.
The gloomy rest we all know.
Personally, many years ago, I used to yell at my kids for their misbehavior, but eventually recognized that much of it boils to my misunderstanding or unrealistic expectation, and if I was my kids I would behave pretty similarly.
Of course, I'm not judging G-d and He must be by definition just, I'm trying to understand His thinking and reasoning (for it is a Mitzvah to follow His ways) and it's important that He not only be just but He has to look just.
So, how do our interpreters see G-d's justify G-d's expectations from Dor Hamabul?