Why should Avatar be locked only to Purity? Avatar is a part of the Life religions but only available to Purity, thus making them genocide machines. Undead is open to all three Death religions allowing multiple options for Undead RP.
Why would the gods of Life not want to empower an avatar of Compassion or an avatar of Tranquility? This would open more options to the RP base that avatars can base off of, without just being siege engines against the open concept of evil.
May be because Purity closes most to the idea of Avatars, destroying all darkness?
I think you're confusing the literal definition of an avatar with the FL definition of an avatar.
In FL It's meant to be an evil hunting machine, not just the embodiment of the life religions.
It's been hit on in this thread already, but from my understanding avatar is just the shorthand we've adopted over the years. The actual qrace is avatar of purity.
This does bring on a new question about having avatars or paragon for multiple religions
It's more a concept that the avatar of purity is and remains what it is, but avatars for others be adopted.