What stops me doing wrong in a materialistic universe?

[devonin]

Nothing stops you from being able to do that. Barring things like the cost of travel, acquiring and training to use weapons etc, you could in fact go kill a tribe of people. You could go rape children, you could go do all of those things.

There is nothing stopping you from acting in any way you desire in the causative sense, barring those things which are impossible due to the constraints of the universe (You can't kill people by thinking at them, or jump 500 feet into the air etc)

What you're trying to fish for is "If there are no objective ethics, if there's no point to life, why don't we do whatever we want?" And the answer is, despite your putting them as answers you don't 'accept' "Because you allow your actions to be limited by your society in order to properly function in that society."

You can make the free choice to go murder somebody for no reason. And while the justice system might be 'wrong' to you if it says you did wrong for moral reasons, it still has the power to lock you up or depending on where you are, kill you for it.

The reason you don't do these things is either a) you respect the boundaries set up by society out of a desire to have others respect those boundaries with regards to you or b) you're more afraid of the consequences of acting in that fashion than you are desirous of acting in that fashion.

To use your exact formulation "If there is nothing beyond this life then why should I not harm others?" the answer is "No reason, but you can't complain if someone does it back, and then some."