Recently, I was reading some arguments about Fermi paradox and aliens and so on; also there was an opinion among the lines of "humans are monsters and any sane civilization avoids them, that's why Galactic Zoo". As implausible as it is, but I've found one more or less sane scenario where it might be true.
Assume that intelligence doesn't always imply consciousness, and assume that evolution processes are more likely to yield intelligent, but unconscious life forms, rather than intelligent and conscious. For example, if consciousness is resource-consuming and otherwise almost useless (as in Blindsight).
Now imagine that all the alien species evolved without consciousness. Being an important coordination tool, their moral system takes that into account -- it relies on a trait that they have -- intelligence, rather than consciousness. For example, they consider destroying anything capable of performing complex computations immoral.
Then human morality system would be completely blind to them. Killing such an alien would be no more immoral, then, say, recycling a computer. So, for these aliens, human race would be indeed monstrous.
The aliens consider extermination of an entire civilization immoral, since that would imply destroying a few billions of devices, capable of performing complex enough computations. So they decide to use their advanced technology to render their civilizations invisible for human scientists.