Correct me if I'm wrong, but weren't Skynet's "motives" always left pretty vague?
Explanation (audio here):
Reese: Defense network computers. New... powerful... hooked into everything, trusted to run it all. They say it got smart, a new order of intelligence. Then it saw all people as a threat, not just the ones on the other side. Decided our fate in a microsecond: extermination.
...
The Terminator: The Skynet Funding Bill is passed. The system goes on-line August 4th, 1997. Human decisions are removed from strategic defense. Skynet begins to learn at a geometric rate. It becomes self-aware at 2:14 a.m. Eastern time, August 29th. In a panic, they try to pull the plug.
Sarah Connor: Skynet fights back.
The Terminator: Yes. It launches its missiles against the targets in Russia.
John Connor: Why attack Russia? Aren't they our friends now?
The Terminator: Because Skynet knows the Russian counter-attack will eliminate its enemies over here.
Thanks. The "saw all people as a threat" bit in particular seems to fit the "figured out the instrumental drives for self-preservation and resource acquisition and decided to act upon them" explanation, especially given that people were trying to shut it down right before it took action.
http://www.businessinsider.com/musk-on-artificial-intelligence-2014-6
Summary: The only non-Tesla/SpaceX/SolarCity companies that Musk is invested in are DeepMind and Vicarious, due to vague feelings of wanting AI to not unintentionally go Terminator. The best part of the article is the end, where he acknowledges that Mars isn't a get-out-of-jail-free card any more: "KE: Or escape to mars if there is no other option. MUSK: The A.I. will chase us there pretty quickly." Thinking of SpaceX not as a childhood dream, but as one specific arms supplier in the war against existential risks, puts things into perspective for him.