Of course not. It is never right to do wrong in order to do something we feel is good. Murder is still murder, even though it might be doing something we construe as "right." Humans do not reserve the right to take away other lives. That is wrong by anyone's standards.

Also, please keep in mind that God does not take this major step in destroying the human race more than once. He did not and will not destroy all of humanity in a flood again. In all the times he called the Israelites to ruin an entire town, he does so for the same reason: the town was too far gone for anything to bring them back from their injustice. He has never, at least not in my readings of the Bible, killed an entire town/race/whatever it may be in order to bring more people into heaven. That is injustice in its highest form.