I think that as creatures expand in numbers, they also need to expand in territory. I think the main reason that humans are willing to spend the resources to venture out to map new territory is to pave the way for exploitation of the resources they find. I don’t think aliens will be any different.
Some people suggest that there are discovery missions that are purely in the interests of science. But I once again say that the reason why we engage in speculative science is the hope that it will pave the way for some “useful” technology.
There is no such thing as a peaceful exploration mission. It’s all about getting a competitive advantage—either in terms of military strength (space exploration) or technological strength (all other forms of exploration).
If an alien makes it to earth, we would do well to heed Hawkings’ warning. They aren’t here to make friends. They are here to map the area and see if there are any resources they can exploit.
The amount of resources necessary to travel interstellar space is beyond imagining. I don’t think a civilization would put the resources together to do that without a serious plan for exploitation of whatever they find.
However, I doubt that we will see any aliens, ever. As I said, the resources necessary to create an interstellar mission are prohibitive. It would probably be out of desperation that any civilization would try to do this. Maybe their planet was dying or something. They would come loaded for bear, to make sure they would not be turned away whenever they got to where they were going. It’s a one way, one chance trip. It either succeeds or everyone dies.
So, unlikely to ever be faced with this scenario, but if we are, we would do well to bulk up our defense and be prepared for all out war.