Why would Allah destroy some cities ( like the people of Aad, or people of Thamud, etc.. ) because they didn't believe .Don't people have free will?

I was educating myself on the different stories of the prophet and in many of these stories, after the people still didn't believe in the words of the prophets (despite many signs), Allah destroyed the city. I was wondering how would I respond to someone who asks:" why would Allah destroy these cities just because they decided to not believe? I thought that this was the reason why there was a day of judgment so that after death Allah will decide if you go to Jannah or not based on how you lived your life. It just seems that Allah is not so merciful if he decides to destroy cities because they don't believe. And why wouldn't Allah destroy the cities of today, since they are much more corrupt now than before?"