If they don't exist, end of question.
If they exist, and have the ability to get to earth, their technology is way ahead of ours. They wouldn't have to watch our planet for long to know we can't be trusted. If we don't even treat each other and other earth bound life - animals, plant life, the environment - with respect, why should they think we'd treat them any better?
They'd destroy us rather than take the risk, or at least impose a quarantine.
If they were less intelligent that us, we'd exploit them, just as we do everything else.
Despite all the hopes of some that there would be life on other planets, quoting the billions of galaxies out there, the chance of finding planets in "Goldilocks" zones like ours is pretty remote.
Everything is just right - not too hot, not too cold, the sun at the right stage in its development, lots of water to keep temperatures stable, tides to keep the oceans clean, protected from cosmic rays by a magnetic field, a large moon to stabilize our rotation and stop the planet wobbling (plus it's other function of providing most of the tidal action), in a dark part of the Milky Way so that our astronomers can actually see the universe, and far enough away from the black hole in the middle to avoid all the issues it could cause, a useful fossil fuel base left behind by previous life and just lying around so industrial man could get a leg up the ladder, along with a plentiful supply of metals, and even uranium so we could have our atomic industry - all sheer coincidence of course, and not an ounce of intelligent design involved in any of it.
Garbage!
I'll believe in aliens when I meet them. Meanwhile there are two supernatural alien species who are very much aware of us - they're called angels and demons.