People put way too much stock into the positioning on that chart. It's a pretty representation of the cosmic forces, not a math equation. There's evidence of pretty much every one of those forces being mixed with each other in some capacity, there's no hard rule for any of them being some kind of magnetic opposite to another.
Forsaken holy priests feel the burn, sure, but Calia was made through some kind of combo of Light and Necromancy and doesn't seem to be in constant agony so clearly it's possible for the two to co-exist.