Soccorro: Just imagine, we will have contact to another species one day... I swear they would just put us in quarantine and then destroy the crap out of us after having visited us for 3 seconds.
tinyE: I watched a thing about alien invasion on the Discovery Channel the other day and they raised kind of a creepy point. Most pacifist like myself would insist a much more advanced civilization would have no reason going around blasting lesser advanced civilizations to hell yet history shows that is EXACTLY what happens. Just two examples, Ancient Rome and Nazi Germany. These two were eons ahead of everyone else in terms of tech and what did they do with it? They slaughtered people! The good news is and alien race that gets here will no doubt have some sort of super cannon that will wipe out the planet in a micro second thus giving us all a quick death. Gotta accentuate the positive! :D
Using our own history as an example is inherently flawed and in my opinion not worth much, yet it is the best example we got. I think it is pretty hard to guess how such a civilization would react to us. When I was a child\youth I was of the opinion that a civilization advanced enough to travel in space would be advanced enough socially\emphatically to be peaceful instead of aggressive. However when I got older I really found no reason to assume that peacefulness is necessarily a logical progression for an advanced enough species. I agree with Jared Diamond in that we should not send out deliberate attempts at communication to potential civilizations nearby, just to be sure. The possible danger outweighs the possible gain by far in my opinion.
I don't think your examples of Ancient Rome and Nazi-Germany were very good however. Nazi-Germany was not "eons" ahead in tech, they were ahead in rocketry as far as I remember, but America for example was far ahead in nuclear science. And while Ancient Rome were guilty of countless crimes against humanity, they didn't go around slaughtering people left and right. Their usual mode of operation was assimilating the conquered territory into their empire (or giving them self-government with limits). You could even make the case that many people were better off with Roman peace and civilization than with what they had before. (What have the Romans ever done for us does make this point eloquently:
http://www.youtube.com/watch?v=9foi342LXQE&feature=player_detailpage#t=41 ) Certainly Roman peace does seem better than the endless warfare of Celtic and Germanic tribes. (But personally I would prefer to b a part of a remote hunter-gatherer tribe than a hierarchical civilization like that of Romans or Celts.
Ancient Rome only resorted to massacres when they encountered significant repeated resistance like in some Greek and Jewish cities (with the exception of course of being Carthage.) What the Romans did towards the Jews and Carthaginians do deserve the label ethnic cleansing, since they sought to eliminate these peoples from their original homeland (by massacres, slavery and exile.) But unlike the case with Nazi-Germany, the Roman genocides were not motivated by ideology.