Mike mentioned this on TPB as the 'question of questions,' but I'm sure this in the books somewhere. Wracu find them painful, for reasons that are hotly contested. One interpretation involves the fact that it's not just places where atrocity wears thin the fabric of the onta. As Wutteat shows, it's beings as well. Wracu, some argue, are demonic in some respect.
Another interpretation turns on the way morality is intrinsic to the ontology of the World. If you look at Chorae as 'logic bombs' designed to obliterate violations of code, then you can chart antipathies to Chorae according to different kinds of violations. Thus the difference between Schoolmen and Cishaurim. Wracu are not simply Inchoroi abominations, they are Inchoroi abominations possessing souls. Like the Cishaurim, they do not so much violate the 'letter' as the 'spirit' of natural law. Chorae are 'ontological stressors' in the latter instance.