We have seen in a number a Hollywood movies that how ambitious aliens come and bring about destruction of the earth. They are shown to possess ultra modern technology that humans cannot match, but still humans somehow manage to save earth from their wrong intentions of conquering earth or destroying earth. But, that is only the happy ending in a movie. The main idea behind is that we have created the notion of some extra-terrestrial life who is mean and wants to destroy earth for his/her desires. We are always afraid of such beings as they are portrayed to have intent to conquer earth.

Are they really a threat to earth? Or, is it the humans themselves who are the enemy of the earth.
I will present the arguments from both the sides and let the audience decide.

What can possibly be the motive of aliens to venture towards earth and their assumed intentions to conquer earth?
Like humans it might be their appetite for natural resources, they would know only too well that a civilization of our size would have used a substantial portion of our own resources, leaving them with very little to fight us for.

On Earth, proven reserves of natural gas total 130 billion tons. Titan, one of the moons of Jupiter has on its surface many dozens of lakes filled not with water or even water in the form of ice, but of liquid methane and ethane, each of which individually having at least as much energy as our own proven reserves. The Oort Cloud surrounding the solar system, is rich in iron, nickel and water ice, and is a truly immense loosely spherical mass of pre-planetary material. It is estimated to contain as much material as all the planets and the Sun itself combined.
Hence, aliens coming for earth's natural reserves seem very unlikely.

Other reason could be that they needed environmental conditions similar to Earth, which might have ceased to exist on their own planet. They may be on the lookout of alternative settlement like Earth. But to assume that a species would venture forth with the same hostility and indifference to life they had centuries previously simply doesn’t make any sense. They, like us would have gotten to a point where they realized that their own existence was imperiled as their natural resources dwindled. A change of heart and mind would be thrust upon them.

Other reason could be for their hunger for power and supremacy over humans. Or it could be that they are also on the lookout of alien species as part of their science and exploration studies. This seems to be a valid point as it matches with our viewpoints as well. But, this point doesn't prove that aliens actually want to conquer human beings.

Are humans really the enemy of the Earth?

Humans are causing a lot of environmental pollution which have a long-term impact on the survival of life-form on the earth. After industrialization, humans have polluted Earth everywhere by every possible means. Humans have emitted lot of greenhouse gases which are already causing global warming. It is estimated that in near future, polar ice-caps will melt and most of the land near sea-shores will be submerged under oceans.

Also, humans have created nuclear weapons which can destroy the Earth many times over. If such weapons are used, it will surely wipeout life from earth. There is world politics and greed of some of the capitalists. Because of this greed for money and power, it might some day actually lead to war and use of such weapons. Then there is issue of terrorism who might get hold of nuclear technology and use it for their wrong intentions.

But, this is an unimaginably vast universe, it is impossible to think of all possible possibilities. There might be some reason which might still be unthinkable to humans and might make great sense for aliens. That possibility might motivate aliens to come to Earth.