So..... humans are the most miserable, most twisted cancer ever to evolve on this planet. In fact, it's hard to think of an alien civilisation that could possibly be worse than us. We're sick, evil, twisted parasites.
So..... I think it's safe to assume that we're gonna be the main cause of 'death' of our planet. Question is: how? Nuclear? Insatiable thirst for knowledge? Genocide(religious/racial/etc)? Utter madness? Chemical warfare? Robots?
We're not fit to be called animals. And some of you cretins here are almost as bad as those fucked up Islamic 'ISIS' crowd in the Middle-East. Not quite but too many of ye were dropped on your heads at birth and used as piņatas. And we (as a species) hilariously think we're intelligent.
All the above is of course assuming that some giant asteroid doesn't fuck us up the anus first.
Dinosaurs lasted a 100 million years or so. We've been around a couple 100,000, less even.
*** I really should have included an option for "All of the above"