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. .... Oh, and extraterrestial intelligent life does exist. The real question is, if we ever make contact or see proof.
You say " ...extraterrestial intelligent life does exist" and then say "The real question is, if we ever ... see proof." How can you say something exist but you have no proof (apart from faith)? So... All you said there are assumptions without proof, on the other hand there are real and scientific proofs that we are destroying our planet, therefore that's the real danger, anything "intelligent" happening outside of earth is just indagation.
As for the intelligent alien life, I don't deny but I don't agree, we still don't know. For that I recommend you research about the Fermi Paradox, which points several different explanation about that. One possible reason simply points out that we are indeed alone, therefore your assumption is wrong, but one of my favorites is the Zoo hypothesis that I described in my previous post (it's just my favourite because I find it fun, they all have no proof).