Short & Sweet:
* "We're the only species that can destroy the world, and we're the only ones that think we shouldn't."
* "Progress: The art of moving backwards faster."
* "We're the smartest species on Earth, but we're also the dumbest."
Slightly Longer, but Punchy:
* "The human race: A bunch of monkeys with opposable thumbs and delusions of grandeur."
* "We're so advanced we can create a weapon that could wipe out all life on Earth, but we can't figure out how to stop arguing about religion."
* "We're the only species that builds a giant spaceship to escape a dying planet... only to take our problems with us."
Bonus:
* "Mankind: The ultimate paradox. We're capable of both incredible beauty and unimaginable cruelty. We're both the architect and the arsonist of our own destruction." (This one is a bit longer, but it's a bit more poetic.)
Ultimately, the best statement for you will depend on the specific tone and message you're trying to convey.