I really don't think any extraterrestrial would want to destroy Earth.
If you are advanced enough to fly over to another planet in another star system you are probably advanced enough to get your materials from other planets that aren't inhabited or just manufacture them using pico or smaller technology.
What would be useful to an alien species on Earth, besides good conversation with humans? Oil? Water? Gold (or any element)? Soil, slave labor (robots are much better at lifting and doing stuff anyway).
It just doesn't make any sense, if you wanted diamonds there are entire planets or old stars that are made of solid diamond, planets that are much richer in elements that are uninhabited etc.
The only reason I can see an alien species would destroy us was if a terrorist alien went full retard and went on a galactic killing spree, and we aren't being monitored by anyone. I think it is more reasonable that an alien species would make decisions with a hive-mind like mentality. Communication would be instant between everyone, and intelligence would be high enough through augmentation to process loads of information. People would hear about any going-ons in their civilization and probably every known civilization. Protests would ensue if there was a bad decision being made, people would be there with the means to stop anything stupid from happening instantly.
Maybe we are just being watched. Maybe they have contact with some people here? Maybe someone on this forum is an alien in human disguise? I mean androids will probably be indistinguishable from humans this century it wouldn't be a stretch at all to believe an alien species would hide themselves as human. Those UFO sitings don't make sense unless they are just spreading the idea of aliens being there. Aliens would be smart enough to get around Earth incognito.