This does not disprove the existence of God. We don't have ALL the data concerning these events. Perhaps the people being judged were so corrupt and sick from their...
sinfulness that it was the merciful thing to do to end innocents' lives. But only God knows for sure and He does (being God the Creator) have the right to make such...
decisions whether we like them or not. Does this sound reasonable to you?
If the person asks where's the proof for God/Jesus/Christianity, go here.