The world's economic and political rulers are generally loath to approve any project whose object is to make contact with extraterrestrial civilizations. The obvious reason is that such projects do nothing to realistically further their own earthly aspirations. But it might also be because, somewhere inside, they are considering the ramifications of contacting an advanced civilization and do not want any competition.
Carl Sagan believed that any civilization which managed to survive beyond its discovery of nuclear power and creation of nuclear weapons, then moving forward into interstellar travel, would have to be some sort of enlightened, benevolent race of angels. They would go from system to system, exploring, studying, wanting knowledge only for its own sake, taking the greatest care not to interfere with any native life they discovered. But who is to say that the survivors of the nuclear age would not be the most hardened, brutal, vicious individuals, the ones who were callous and bloodthirsty enough to crush everyone else down and stand astride the remains? Cruelty and treachery select for themselves.
Regardless of this and any Independence Day-type hysteria, I believe that were an advanced alien civilization to physically contact ours, they would end up destroying us. It sounds paranoid. Why would they do that? you ask. Why would they not?
Christopher Columbus did not cross the Atlantic with the goal of exterminating the Arawak. Spain and England did not send explorers to the New World with a mission to bring about the collapse of native civilizations. But this is what ended up happening, because of the huge power differential involved. If you read accounts of explorers when they first reached the Americas, they talk about how friendly the natives are, how pure and good of heart they are, ready to share anything... the indigneous are painted in the most glowing terms. This lasts until the explorers realize they are far, far more powerful than the native peoples. Then the slightest infraction is visited with absurdly extreme punishments, societies are shattered for specious reasons, whole peoples are enslaved as an expendible workforce or exterminated outright. Once the European explorers understood that the American natives were so easy to victimize, the natives became to them subhuman, not even afforded the consideration of beasts. They were considered a plague, a pestilence to be brushed aside. The ultimate result was the near genocide of the native populations of North and Central America and serious depredations upon those of South America. This was not Columbus' original intention. He just wanted to find a more economic route to the Far East. These things just happen when there are vast differences in power between two parties.
The pattern continues today. The death of 3,000 people in New York City, in the very heart of the most powerful civilization on Earth, is considered the starkest tragedy. The death of over 1,000,000 people in Iraq at the hands of that civilization, both by immediate attack and in the ruined country's decline, warrant almost no mention. They are subhuman, they are somebody else, they are statistics that do not matter. The strong always reserve the option to destroy the weak if it is for their purposes expedient to do so.
But with regard to our hypothetical alien encounter, there is some reason not to worry. Carl Sagan also believed that if an extraterrestrial civilization reached the Earth and decided they wanted to destroy us, the gap in technology would be so vast that there would be absolutely nothing we could do to stop them... so it's pointless to concern ourselves about their intentions.