Colonization? A hunt for additional living space? If the former is something aliens do, then they won't wait to hear from us before doing it. The British, after all, didn't begin their colonization of Australia because they had intercepted some aboriginal communications
Other suggestions about why they might visit include forestalling competition in the Milky Way marketplace, proselytizing, or just learning more about us. It's not clear that any of these goals requires "killing us," of course, but the logic is wobbly anyway. Any beings that actually could come here will be far beyond us in technological accomplishment. Imagine if you could visit the Neanderthals. Would you worry about commercial competition? Would you give them bibles? Remember: these are (nearly) the same species as you are. The aliens won't be. I dare say you wouldn't try convincing porpoises to join your church.

and the reason I say - we got MILK.