Huge subject, and I can only hit on a couple of thoughts that come to mind.

You mention the possibiity of a moral or ethical code arising from biology or evolution. There may be some truth in that, among species like ours whose evolution enters the phase of abstract social constructs. Evolution and natural selection work to promote the longterm survival and prosperity of species (not individuals), and so I'd say that codes of behavior in the social environment should arise that promote the general welfare. Murdering a member of your own species should naturally be considered wrong, because unrestrained intraspecies killing undermines the health of the species as a whole (sad to say, within limits intraspecies killing may make the species healthier after it's advanced beyond the capability of other species to "thin the herd"). Sexual morality seems to be rooted in biology too. Prior to birth control, faithfulness in women had the practical benefit of allowing men to devote their energy to raising their own offspring(promoting their own genes within the species gene pool), which is positive in evolutionary terms; cuckolding forced a male to expend his energy promoting the genes of a competing male.

This implies a moral code focused within the species, defined in terms of the species. Note how we define killing humans as wrong, but generally not wrong to kill other species. In the scenario of contact with a nonhuman intelligence, while our respective moral codes may overlap with respect to their rules and structure, would we be likely to extend them to apply to the other? Think "Starship Troopers" and "bug killing". Think of the difficulty a lot of humans still have recognizing other groups of people as humans and applying the Golden Rule.

Maybe this is why the aliens are staying away. We're not ready yet treat them as we treat ourselves.