Quote Originally Posted by dragoelete View Post
you could technically say the same of the voidsent cause you need both light AND dark to truly be termed as alive so that doesnt say anything...
Likely because when things go too far into the chaos of darkness every element that makes up a living organism loses its distinction and merges into one big mess. Whereas with the light everything stalls to the point the aether of the healing magic simply cannot treat it because the aether in the healing magic gets effected and comes to a standstill as well, or simply cannot accelerate whatever combination of elements is needed to repair the damage because the individual elements were rendered inert in terms of activity.

Which would likely explain why Sin Eaters hunt for the aether of living beings. They need regular intake of high activity aether in order to run. Otherwise they would eventually completely cease activity altogether. In the same healer role quest series it gets revealed that the Sin Eaters that can be healed are ones that were converted from other beings, which would suggest that they still retain a high enough level of active aether for healing magic to work on them.