Seems like that Athiests and Agnostics are all over this one… Don’t be mislead into thinking the answers to these questions lie in some simple dismissal of the type you’re being advised to believe here. Rather, the true answer lies in actually understanding what was going on historically at that time, who the people being destroyed were, what it is they were doing which was of such grave offence to God…
In many cases these cultures were practicing brutal “religious” practices, so much so they even offended contemporary pegan cultures who also executed similar slash and burn attacks against those who hold these beliefs with out any prompting from the one true God. Among these was the practice of super heating a metal statue of a bull God with out stretched hands, and placing a new born baby in it’s arms to be burned alive. A practice which they believed would bring a good harvest the next year.
A careful reading of the OT will revel, that it is possible for a culture to become so fundamentally coorpupted that it can’t be fixed by anything less than extreme measure. This isn’t to say God didn’t give them any chances to repent:
God kept an open line with the Ammorhites, which is witnessed by the fact that Baal was a priest of the one true God. God did communicate with him, and others and most certainly let them know what was coming as he would several hundred years later when the Jews them selves would become so coorupt as necessating similar extreme measures.