The difference between the original movie and the new one
It's been a while since I saw this movie, but from what I remember:
In the original movie an alien civilization had created a robotic police force programmed to destroy everybody who tried to bring mass destruction weapons into space, including their own creators. Once they had been activated, there was no way to stop them. But being just machines, the robots couldn't care less as long as no weapons left the worlds where they were created.
The lesson of the movie appeared to be that nobody was strong enough not to go to war if they alone had to make the decision, they needed to be forced not to act in a destructive way.
Klaatu wanted to warn people for their own good.
In this movie; the robots are just the servants of another race that wants to find out if humans are too destructive to be allowed to live, and if not, they intend to save all species on earth except for humans.
Klaatu was there to judge the human race (the old humanity on trial concept combined with environmentalism).
I think this movie would have been better if they had stuck with a version of the concept used in the first movie instead.