Are we alone in the Universe?

For ~50 years, the Search for Extra Terrestrial Intelligence (SETI) has been scanning the galaxy for a message from an alien civilization. So far to no avail, but a recent breakthrough suggests they may one day succeed:
Horizon joins the planet hunters who've discovered a new world called Gliese 581c [ http://www.youtube.com/watch?v=PmTjAmop688 ]. It is the most Earth-like planet yet found orbiting around another star and may have habitats capable of supporting life. NASA too hopes to find 50 more Earth-like planets by the end of the decade, all of which dramatically increases the chance that alien life has begun elsewhere in the galaxy.

We must be assuming that aliens are peaceful in order to search for them like this.
What if they're not ?

"We spend our life in 3 spatial dimensions, confident that what we can see with our telescopes is all there is, ignorant of the possibility of 10-dimensional hyperspace. Although these higher dimensions are invisible, their ripples can clearly be seen and felt. We call these ripples gravity and light."