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?"
Asalamu Alaykum, God can destroy his own creation when he wants to. He still destroys cities today. Allah has the worst punishments but he is also the most merciful. He says he will not destroy everyone who sins in this life because if he did there would be no one left. Allah decided to destroy Aad and Thamud as a punishment, who are we to question him? and for the people who die or are killed but are innocent Quran 2:154-157 is for them. Everything that happens in this life is a test, nothing really matters except our deeds.