I think part of his issues stem from his projection of Catholicism onto The Light. While they share some common themes as well as some cautionary similarities. They are not the same. Villifying the Light is not villifying Christanity. Blizzard has a pro-Christanity (or maybe more accurate to say pro-revistionist occult centic christanity) narrative in Diablo.
Thad gets triggered by any criticism towards Catholicism, even if it’s just a projection and not what’s really happening. All religious belief can lead to dogma, and radicalization, and violence. Thadeus is the only one here who, for whatever reason, can’t accept that universal truth.