In a ominously informative animated essay, the ever-philosophical design house Kurzgesagt explains why the arrival of extra-terrestrial alien life upon Earth would be bad news for all of humanity. This newly arrived civilization may require a specific environment to survive, which may not be compatible with the existing civilization. If the newcomers are stronger, then their very existence will present a great filter of danger under which they thrive while the original inhabitants die off.
Once a species takes control over its planet. It’s already on the path to self-destruction. Technology is a good way to achieve that. It needs to be something that’s so obvious that virtually everybody discovers it and so dangerous that its discovery leads almost universally to an existential disaster …once a civilization is powerful enough to change the composition of its atmosphere they make their planet uninhabitable 100% of the time.