Atheists: Don't you think it's cruel to convince someone who believes in god that there is no god?

Surely it's only fair to hear both sides of the argument. Usually one who believes in God does not take into account arguments against their religion, rejecting them with paradoxical religious answers (e.g. the bible must be true because the bible says so). What would you rather happen, a man is mugged and only the testimony of the mugger is taken, or everybody's testimony?