Chances are that not only they exist, but that they are also hostile racists.
Just like us they had to survive the evolution process, possibly on a more hostile planet, fighting more hostile animals. That alone can make them smarter, deadlier and more coward than humans.
The first alien Christopher Columbus to arrive on Earth's shores is likely to be the last alien we will ever see.
I personally disagree. Life on earth existed for a long time and throughout history there have been accounts of alien encounters and UFOs (including ones that got out of the ocean). They're in famous paintings, they're in books the Vatican tried to exterminate and burned alive those who wrote them. Of course one could say that all those historic recordings might be just someone's imagination not a record of something factual. There's no way to argue pro or against but thing is there are tons of such records. Not just that but archeological findings that make no sense.
On top of that, considering the time since we exist here, and by extension life on earth exists, there was no alien-produced extinction so far.
Now, leaving all those hard-to-prove things, I think you're missing something when you compare aliens with Columbus. Columbus went searching for India and had a financially motivated plan. When he reached Americas the empires took over because they had material gains to obtain. At that time there were certain mentalities and interests. Today same thing happens - though in a different form - with natives in Amazonian forest who get extincted by oil/wood/etc. companies so they can exploit the areas (look on
Google earth engine for amazon deforestation over past 20 years - zoom out!).
However, that is us, humans. Saying aliens are expected to be like that is a generalization without foundation. Assuming aliens would be the same is like assuming ants are like us or wolves are like us. And no, animals don't do it because they're retarded - they could easily kill more than they need - but because they are living in a certain equilibrium with their environment. We are the only species that screws up the environment and others and we don't all do it. As a matter of fact saying humans extinct eachother is incorrect.
Some humans extinct others. Not everybody is an invasive, destructive maniac wishing war.
Another aspect is what happened with Columbus was an invasion/war/conquest not an extinction. When he discovered Americas the interest was not to extinct everybody there but to control/enslave them to work for the empire. Same shit happened everywhere not just with Columbus. It happened because that's how those times were not because of a superior vs inferior species contest.
Extinction in nature literally does not occur. If it does please correct me. I simply can't think of any species that extincted another one. Even in predator/prey situations the predators do not produce extinction of the prey because they need it (equilibrium). Plus the Columbus example is not species1 vs species2 but a conflict within same species. Even ages ago, "less evolved" people used to live in equilibrium. American Indians respected animals and had a very balanced life without destroying their environment.
Moreover, we are already technically evolved enough to destroy all life on earth (Nukes) but we don't. Nobody uses nukes against others that are weaker. They use standard warfare. Because they don't want extinction they want control.
"Just like us they had to survive the evolution process, possibly on a more hostile planet, fighting more hostile animals." - contrary to you I think survival is achieved through cooperation not fighting. This is a fundamental reason why we exist - because we mainly cooperated. This cooperation exists even between species (2 species work togheter because they benefit from eachother). So I think for aliens to manage to achieve such technological greatness and such a high degree of knowledge and understanding of the universe they must have cooperated quite well, otherwise they would have self-destructed. As a matter of fact I think there might be a critical point where you discover the "nuke" - you either destroy yourself or you learn to cooperate and stop fighting.