Yup, the theory goes something like: at least
10% of stars have planets, roughly 10% may have a planet in the goldilocks zone, roughly 10% of those would have a planet there which is in an age range appropriate for life, and perhaps
10% of those happen to have evolved life, and perhaps
1% of those have intelligent life (the last two numbers are speculative, the previous ones are based on science)
By those numbers our own galaxy would have 40,000,000 planets with life and 400,000 alien civilizations. Of the stars we can see from earth, 10,000,000,000,000,000,000 would have intelligent life. Even if that math overestimates the occurrence of intelligent life by a factor of 100,000 we still shouldnt turn our planets stereo up too loud, we apparently have at least 3 neighbors
I'm not personally convinced that earth is the only planet with life in the Sol system. We've barely even tested he moon, much less other planets.
I think if there is other life... I'd like to think they would not need to kill to live
This is biologically plausible. Photolithotrophs, organisms that eat only rocks and sunlight, are the foundation of all life on earth. With the inherent biochemical inefficiency of 'higher' life forms, the photolithotrophs stand as the replenishing source of biological materials that keeps all other life from eventually starving. Something to be grateful for next time you see a blob of goo on a rock.