What is nice with alien is that we really don't know. So the sentence above tells much more about your pessimism than about aliens.
I think that's the point. When you don't know anything about the motivations of an alien race that can easily wipe out our species, why would you take any chances?
Pretend for a moment that I have a Button. The owner of that Button can push it and eliminate every person on the planet besides the people that owner didn't want to kill (friends, family, city, state, country, whatever)
Would you be an advocate of giving that Button to any random person on Earth?
Any race of aliens capable of interstellar travel probably has that Button. So while (I assume) that you wouldn't be okay with giving that Button to any person on Earth -- you'd be okay with having an alien race that we don't understand even a little bit in possession of such a thing.
Unless of course you believe they're hiding inside Europa...
We are all aware of the reasons to be pessimist, because of all the books and movies, but there are reasons to be optimist too: advanced technology may come with advanced ethics, for instance.
You criticized the original poster's sense of pessimism for holding the only parsimonious, evidence-based and logical position on the subject.
Have whatever fanciful opinion you want to on the subject, but pretending it's based in anything but wishful thinking is naive.
Hawking has said that we need only look at the history of inter-species relationships on our planet (let alone race relationships) to get a hint of what exopolitics might look like. He goes on to mention that in terms of the search for alien life, we'd do best to keep our heads low.
And there is a reason to believe that they probably won't be - they will be the product of Darwinian evolution, just like us, which is really not a very nice thing.
I'd rather compare alien encounter with something like Marco Polo going to China: he didn't wish to kill them all, was impressed by some achievement and inventions but was also disgusted by the Chinese diet. Came back and wrote a book, and it was so foreign most readers didn't believe him.
Because of the properties of exponential growth, there would be nothing left over for us. If two organisms double their numbers 250 times, their numbers would roughly equal the number of atoms in the observable universe, which means there would be none left for anyone else.
The only hope would be that they either don't reproduce, which seems unlikely since they would be the survivors of a process of Darwinian evolution just like any other critter, or that they're Space Vegans with a moral code against exterminating other species. We really don't know how they would think.