The portrayal of aliens in movies varies so much its hard to decide what might actually happen if we were to encounter some visiting species from another planet come to ours. There's such vicious monsters as the Aliens from Alien or the complete abomination that is The Thing but then there's the cute and cuddly E.T. and the wise  Klaatu of The Day the Earth Stood Still. I would like to believe any species advanced enough to travel to other systems would be similar to us in the way of looking to communicate with and study things not familiar to them. Although I suppose the argument could be made that they are like us in that they are power hungry leeches looking to absorb the planets resource after running their own dry. Either way i don't see an alien species all the way to our planet simply to destroy it or for the soul purpose of killing all of us then going on their marry way. I'm just a lowly sci fi enthusiast speculating though, mean while the big guys are doing the real thinking. I a recent discussion, I stumbled upon while browsing the web, Stephen Hawking made his own theory. He speculated that if and alien race were to visit planet Earth it would be for Conquest, similarly to the Spanish coming to the new world. The would be extremely hostile and take what was our as their own, either for colonization or simply for resources as i mentioned before.
I would like to believe he is wrong or that hopefully we are the advanced beings stumbling on to the planet of a slightly more primitive race and are nice to them. Show them our awesome tech and BAM new alien buddies.