You must have just kept interrupting them and not letting them make their point. Because any JW could answer that the difference between the destruction at the Flood and at Armageddon is that Jehovah has since sacrificed Jesus for our sins and we won't be continuing on as sinful humans after the next destruction of the wicked, but must apply that sacrifice and then we will have our unintentional sinfulness removed, which never happened to Noah's offspring.
And then, "Well, why did God do it back then if He hadn't sacrificed Jesus yet?"
Because the world was TOO wicked and it was part of preparing the way for Jesus to be born into the world.
It doesn't even have to make sense to the brain, what JW's will say. But they know how to say it.
If the dialogue wasn't an exercise in just constantly barraging them with piled on thoughts, then it was your exercise in creative writing, because I am confident that JW's today, no matter how bad they have gotten, will think they could answer such questions.