Pretty much any new technology has two uses, benevolent and violent. Having followed some of what Elon Musk has done for quite some time now, I tend to see him on the benevolent side. I would rather he be working on it than Gates or the Military branch of DARPA, or the CIA branch of Google.
If it was Gates we would have a very serious concern about viruses, after all he wants to decrease the world population. If it was DARPA then we would have to worry about situations as presented in the movie "Universal Soldier", and if it was the CIA and google then robotic Manchurian candidates.
The genie is out of the bottle there is very little we can do to put it back in the bottle.