I have never believed in the kindness of aliens. In any case, the question is the province of evolutionary exobiology rather than the extrapolation of problematic values of human decency. Stand aside, heart-on-the-sleeve SETI astronomers and stop transmitting to the aliens!
Of course, there is one bunch of aliens actively hunting for signs of life out there: namely ourselves. It's believed that within a century we will have orbital telescopes able to image the details of extrasolar planets just like spy satellites. Of course, our intentions towards any aliens we find will be nothing short of benevolent, won't they? (I think it was a year ago I was watching that 3D epic Avatar about Pandora).
However, we don't need any such hi-tech observatories to detect alien life: a simple spectrum of the extrasolar planet's atmosphere showing oxygen absorption lines would suffice. How hard is that? Just a few decades away.
Take the earth. It intercepts 0.75 * π * (6 * 106)2 kilowatts from the sun, and with an albedo of 0.3 reflects 30%. That's around 25,000 Terawatts if you work it out. If you can detect that from light years out, you have your spectrum and you know someone's at home.
By comparison, the Arecibo telescope in its radar mode emits 1 Megawatt (I know it's collimated and coherent). Still, if you're sufficient light years away and you can detect Arecibo I reckon you can also detect earth's spectrum. Add us to the list.
So I still think it's a bad idea to holler to those aliens out there: it's capabilities not intentions that you have to worry about. But until we start using Exawatt lasers I agree it just doesn't matter.
