I'm an absolutely avid fan of Mr. Hawking. I've read and kept up with much of his work. Hell, I wrote a bio on him back in high school. (yeah, I was that nerdy) However, he isn't always right. There's been times when he adamantly defended one point of view, only to reverse that stance years later when new theories/evidence arise. (a very admirable trait) He even reversed his view on his own theory. He, and much of the scientific community, held the idea that black holes were literal "points of no return". However, he later postulated that black holes radiated energy, later known as Hawking radiation.
The trouble arises when people take anything he says as gospel and are then unwilling to hear opposing views. This happens often around the world, with other, influential people. (religious leaders spring to mind) Point is, just because he said it, doesn't mean it's right. Even he'd tell you to look for an opposing view.
I can't deny there's a possibility that an alien species could be hostile. However, long-held sci-fi ideas about what futuristic space-travel would be like aside, the sheer difficulty and cost of sending a vast majority of a civilization across the vastness of space all but guarantees that ANY other species would not even bother going through the effort of invading. Therefore, I think it's more likely that a hostile civilization would likely just adhere to broadcasting a "stay the hell away!" message out into space. (seeing as they'd likely be xenophobic)
This is all assuming they'd be more advanced than us. Something that's not as likely as you'd think. The other thing to consider is, if they really are as advanced as Mr. Hawking is saying, then it's likely they wouldn't even consider us as civilized. We'd be nothing more than primitive animals. Not worth communicating with.
Now, I know what some of you are thinking, "If we're so insignificant, and they want our 'resources', then they'd just wipe us out and take what they want." That's partially true, but would it be worth the trouble? Would the rewards out-way the costs? Let me explain. Consider the plot to Avatar. Had real-world physics and rules been applied, it really would not have been worth it to go to Pandora and retrieve that "unobtainium". There's the cost of shipping all of that material and personnel to Pandora, not to mention building the vessels used to transport them. The constant production and filtration of the air they need to breath. The fuel needed for all of those vehicles and mining equipment. The energy need to run all of that equipment. The food, hell, even the wages for the personnel. Then, finally, all of the resources needed to send the mined "unobtainium" back to Earth. On top of it all, add in the trouble and cost of fighting the Na'vi, and the costs quickly spiral out of control. Well beyond what they'd make mining.
I know it's a stretch using a movie plot as an example, but it seemed apropos. My point in all of this is, there's no reason to assume an alien species would be hostile. Percentage-wise, I'd think it'd be more likely an alien species would be either indifferent or far, far less advanced than us. It's more likely we'd be the hostile species invading a world inhabited by very, very primitive creatures.
Just my 2 cents. Consequently, I loved that Discovery program. I plan on watching every episode.
