That we are destroying the planet is not proof that we are more dangerous on a scale, that would put us higher than any aliens. The aliens at least would have proven that they have high tech and could strike us down from far away by dropping stones really fast (it worked against the dinosaurs). That is what I would call dangerous. Also, they might just as well come here with an arch ship and attack us, because they already destroyed their own planet, outscaling us twice.
Oh, and extraterrestial intelligent life does exist. The real question is, if we ever make contact or see proof.
It might just be, that we are among the first to emerge. You need heavy elements to have planets and enough stuff on the planet to do chemistry. Heavy elements are not that old in the universe, you need several generations of exploding stars to make them. But even in interstellar dust clouds we can see organic chemstry happening, including so much amino acids that we can see it with telescopes, so the stuff is all there. You only need some lightning and pre organic soup to form stuff that looks like proto cells. While we are very lucky with our planet, it looks like you will have life everywhere where there is stability for a billion years or three. One of those is bound to be intelligent eventually.
Most are not really aware of that fact, but we have no clue how big the universe is. There is a limit how far we can see, but that is not the end, only the limit of vision. The universe is bigger than that. Wich means, whatever number of planets you probably think you can "roll the dice" to see if there is life, your number is too small.