Perhaps the aliens are a race of intergalactic chefs, seeking the most tender morsels in the galaxy to pass across their scaly palates. Maybe we are particularly delicious....
Many of the comments here talk in high terms about non-aggression principals, but these are our moralities. An alien species will have its own moralities, that likely have no relationship to our own. Perhaps they value hunting, conquest, water, who know? The point Hawking makes is valid, and our own history of conquest bears this out. No one in the middle east valued oil until industrial nations found a use for it, and then it was a land grab. Native Americans didn't own the land, until Europeans arrived, etc...
Many of the comments here talk in high terms about non-aggression principals, but these are our moralities. An alien species will have its own moralities, that likely have no relationship to our own. Perhaps they value hunting, conquest, water, who know? The point Hawking makes is valid, and our own history of conquest bears this out. No one in the middle east valued oil until industrial nations found a use for it, and then it was a land grab. Native Americans didn't own the land, until Europeans arrived, etc...