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.
Hawking really hasn't thought this through. Why would they have to colonize any kind of planet whatsoever? If aliens could travel between the stars, then they can also pretty much build any kind of space habitat as they wish. Why search for random planets, when it's much easier to just build a place to live, optimized for your species?
I think it's safe to assume that intelligent hi-tech alien species (including us humans) are relatively rare and very, very far from each other. Say, a thousand light-years between them. I think it will take at least several hundreds, if not thousands of years until we make first contact. Most likely, we will never meet any aliens. Maybe there is just one intelligent species per galaxy - and we are it in our own Milky Way?
The massive distances make any kind of warfare between different intelligent lifeforms pointless.