There is no Being to help guide the actions of man.
How are we going to establish right from wrong?
One might argue for Natural Law but that is a slippery slope. Once you argue that there is inherent right way to live and wrong way to live in nature, then you start sneaking in an intelligence to nature and that will just ruin everything.
But wouldn’t it be good to not kill? Could we not begin there? Possibly. But even there is a dangerous pin upon which to stand. If this marvelous, complicated universe just happened, like snow flakes falling from the sky accidentally forming a snowman but WAY more intricate, then life really has no meaning except to the person who happens to have it. Why should life be better than no life? Why should your life be important to me if you have something (land?) that I really want? There can be no inalienable rights because that implies order, intelligence and meaning, which leads us to a discussion of God, which is something we want to avoid.
There can only be one answer in this type of universe: The one with the most power decides what is right and wrong. It could be an individual dictator, or committee, or even a whole voting nation. We could vote in many of the dictates of any particular religion if we want. And we could vote them right back out again. It will, of course, lead to even greater meaninglessness with a thin covering of legitimacy.
Take, for example, marriage. Back to the true pagans, it had to do with fertility and child raising. In our modern age we have removed that definition completely so our brothers and sisters, or brothers and brothers could experience a type of permanent relationship, legaly binding, that we call marriage. This particular route seems compassionate but this idea of marriage is destined to disintegrate. If a desire to be married is the only requirement for being married and we have removed any ties to our history, then why stop at two people? Why worry about a brother and sister who want to marry? Why shouldn’t all the religious brothers of a monastery be recognized as one giant marriage to give them all of the protections that others have?
The argument can’t be, “That just wouldn’t happen.” What are you going to base a “This far and no further” law on other than right makes right? (There are already legal attempts at similar scenarios of the above to this.) In the end then, marriage will cease to mean anything unless the individuals want it to and have the freedom to do as they please by those in charge.
Now imagine we have formed a society that has bought into a meaningless universe. Imagine further that you are frustrated young man who does not happen to be part of the society that makes decisions and has a voice. You think culture is wrong and you see those you love and their ideas abused. You have no power except to conform.
You are angry, powerless, and have no hope even in a next life. What is to stop you from grabbing a gun or a knife and going on your college campus . . .