Since I have never encountered aliens, it is hard to determine whether they would be friendly or attacking people on Earth. Just due to fear that the humans would attack them if they entered our planet, it is believe that these aliens would start attacking because they would be afraid that earthlings would attack them first.
However, even if the aliens came to the Earth and did not fear us attacking them, I still believe they would probably not be friendly. They would come to take over the Earth. I always imagine that aliens would arrive on Earth because their planet has exploded or can no longer house the aliens on that planet. Therefore, the aliens would come to Earth by force to live here.