I mean, the parallels of Defilers and Healers being opposites is kinda baked in.
One specializes in Healing Magic, finding increased success in casting healing and support magic, giving up many other tools it would otherwise have for the purpose of being selfless and aiding others. The entire point that healers literally give up all their personal power to be able to aid others more is the reason I think it should be Good or at least Non-Evil locked. I simply do not see a inheriently evil character giving up so much personal power (The opposite of being selfish) to the benefit of others.
Looking at the Wiki, Healers literally give up everything except light armor proficiency to attain the powers they have. They only have simple weapon proficiency because of magic staves / Fishing tools, originally that was even taken away from them.
Conversely, Defiler Clerics do not give up as much (Like seriously. They lose 33% effectiveness on Positive Healing spells, where as Healer Clerics lose 75% effectiveness of Negative energy spells, and Defilers can still heal themselves just fine with Negative energy spells? Weird), they lose access to heavy armor but still retain the use of large shields, that's it. They lose basically nothing for their raw power increase. And their power does allow them to "Share" the healing they give to their allies whom are close, but the main focus is simply raw destruction and entropy.
One path is mostly selfish with a bit of useful "Oh, I'm totally helping my friends with my powerful magic." while still retaining most of their personal power and suffering very little drawbacks to their powers.
The other literally basically gives up EVERYTHING, and suffers SIGNIFINCANTLY more drawbacks, for the little benefits it does gain, and all their tools are entirely selfless tools made to aid others.
Whether you wanted the paths to be opposing sides or not, everything about the two paths is literally diametrically opposed to one another.
Everything about Defiler Screams Evil Cleric bent on destruction and wrath. (Selfish, Destructive, Evil.)
Everything about Healer Clerics screams selfless priests/Priestesses that forsake personal power to raise others up. (Selfless, Protective, Good)
That's not even bringing up the parallel that Negative Energy is Entropy Incarnate and literally has no application where it's not destructive, compared to the fact positive energy can injure, yes, but it is used in a multitude of ways to bring benefits to the world.