Because the Anu took our immortality from us, due to the inability of our bodies to filter out the build up of toxins in our bodies (Hence why consuming large amounts of vitamin C is important for a "long life")
So, when we smell these toxins in other species, that would kill us rather quickly, but is easily filtered out in their bodies- we have an innate instinctual aversion tot hem- and an instinctive desire to wipe them out
Funny such a thing is causing the extinction of so many living things
