You forgot to include this quote, which comes right before it:
He suggests that aliens might simply raid Earth for its resources and then move on: “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach.”
As smart as Steven Hawking is, he knows nothing more about ET life than any other person on Earth, so I do not see why we should necessarily take his normative prescriptions over anyone else's. The prediction seems especially dubious when it comes to apocalyptic and doomsday scenarios that Hawking seems to be pushing. The default position of someone who wishes to make a prediction but does not have the knowledge to do so is pretty much always a doomsday scenario--it's easy and sensational. It would be wise just to not get involved in such precise predictions.
It is fun to think about--and I am sure Hawking has thought a lot about it, so I am interested to hear what he has to say--but let's not be presumptuous enough to say whether we should or should not contact ET life.