Sort:  

I think the theory is that by the time a species develops interstellar travel that they'd be able to work together on a global scale. That scale would require cooperation and diplomacy which would likely translate into a general peacefulness towards other (likely lesser) species.

Another way to think of it: Would we go out of our way to make war on bonobos?

We will happily drive a native species from it's habitat if there are resources for us to exploit though.

There are resources a plenty in space. Why waste energy coming down to a gravity well to extract something you can get from an asteroid or comet?