Why did the God flood the earth if we have free will?

I wanted to post this in stupid Questions because I feel like it might be one, but they don't allow religious topics.

I don't understand a whole lot about Christianity, but from my understanding, God did this because the world was so corrupt and evil. However, we have free will, which is a huge part of Christianity, and that's why God doesn't interfere with us today. But if we have free will, why did he do that?