Part of the concern, of course, is that an alien species may be more paranoid than we are.
Assuming that there is no loophole in space travel; that in order to get from point A to point B within any reasonable time scale you have to accelerate to extremely high velocity, then interstellar trade is functionally impossible.
Operating under the assumption of this technological plateau, other races aren't going to know anything really significant that you don't know already, so even trade in pure data isn't particularly rewarding.
But while it's not practical to engage in interstellar commerce, it's certainly possible to engage in interstellar war. Accelerate an object to 90%+ of lightspeed, and it's entirely likely we'd never even know what hit us. Thwack. Dead planet. No more humans.
Now, you could ask why aliens would exterminate us on sight. To which the simple response is: Why wouldn't they?
Our continued existence serves no practical purpose for them, and there's always the risk that someday we might decide to wipe THEM out, so logically it makes sense to get rid of us first.
There's an excellent work of fiction on this topic called "The Killing Star", by Charles Pellegrino and George Zebrowski.