I agree, having no punishments can cause a big difference in behavior of some people when they play games like this.
I do believe though, that in DU there is still punishment. The people who have this behavior (going round eliminating people) are most likely going to be targeted and regarded as high-value targets by governments in colonies or large planet civilizations or even bounty hunters seeking a profit. This will discourage them to commit their crimes as they will gain a very high reputation, which if you are a pirate or murderer, will only result in the destruction of their possessions (ships, possessions on the character). This alone is a big punishment because it is apparent that ships are VERY costly to produce not to mention that you will maintain your rep even after death.
So I'm pretty sure people preying on other players isn't going to be a big issue, you might have the occasional encounter with a person who is bent on destruction but life is full of unexpected encounters, and that is something that we will have to just accept as this game makes up for this problem in so many other areas.
I haven't had the opportunity to experience this game yet but from what I have seen the devs have thought thoroughly about how everything from the RP side to game mechanics is going to work. Hopefully, they reach the full potential of this game and make a space MMO like nothing ever seen before.