You mean Noah's flood? God is God. That's the answer. If you build a snowman in your yard, you can also demolish it. God is God. God judged his own creation as God, and destroyed it. Not much different from the man who destroys his pottery just because he doesn't like it, and starts again with the mass of clay on his potter's wheel.

I could think of much harder conundrums regarding God's ways. Why did He have whole cities of Refaim killed in the book of Joshua, down to the last woman and child, including all the animals? And that is easy compared to why He had his Son sacrificed on the cross for people that in the future would not even believe in Him? What does He see in YOU or me that He would die tortured for YOU/me? Who would sacrifice a son to save a slave?

Questions like these should be asked.

That's an argument hard to lose. Why don't you try that one?