Also, let us not forget that animals in the wild have shown hostility towards their own species as well as others over territory, food, and the like. There is no reason that another life form might not also have those same instincts.
There is also the matter that a species that travels across known space would have to have a damn good reason to do so. Searching for knowledge or simply to contact new life is admirable and the preferred mentality that I would like to be evident if/when it so happens. But I won't discount a desire to conquer and pillage either. Both scenarios are possible.
