Imagine yourself on a long vacation with your friends to any not so populated part of the world and one day you wake up to find the whole city in chaos, people running around shouting for their lives, weather changes at the blink of an eye and the building and trees collapsing and blocking the roads. Then somewhere far you find cyclonic condition and then what you finally see a UFO in front of your eyes. You must have heard of it several times but this time it not a Hollywood movie but a reality you are facing.
Now what comes to your mind is that it must probably be an alien invasion in the city. This is the question that we need to address why just imagining alien supplements the invasion in the city. Why are aliens often taken up negatively?
This is a normal human tendency to regard the unseen powers to be superior to us and as our history of evolution suggests every advanced species has always destroyed the older ones for their development and betterment and regarded the older ones as useless and killing them in huge numbers. Similarly, aliens are portrayed as superior species with advanced technology to allow mass destruction in no time. So their arrival on Earth may mean a loss of human superiority over other species. This will lead to the destruction of mankind in large numbers.
Another reason for humans to feel insecure of the aliens is that the humans have turned really materialistic. We consider the Earth to have best conditions for survival, so there is a huge possibility of aliens turning up to our country and rob us of all our precious property and other essential resources keeping us as their slaves.
Finally, the Hollywood movies portray the aliens as villains to gain mass attention. It is a commercial way of promoting movies to show some thrill and capture the hearts of its millions of audience. Obviously, there is no point making a movie where aliens and humans live together very happily without any destruction as it will have no viewers as well. These Hollywood movies have captured are imagination so ell that we always consider aliens to be destructive species and against the interests of the human existence on Earth.