Shou's personality seems to have done a complete 180. Strengthening Nazrin isn't a bad thing at all, but she's being a complete asshole about it. What on Earth happened?

Well, to Shou's defense, Nazrin was sent to spy on Byakuren and Shou's actions. In a certain way, it's like Bishamoten had no real trust in Shou as a representative and had to keep a mouse around to share every single secret that escaped the temple. Shou made a mistake for she got too attached to Hijiri, but her mistake went relatively unpunished for Nazrin killed the leaked secret with her mice.

She's probably pissed off more at herself than at Naz, for she (most likely) believes that blind attachment is harmful not only to you, but to those you choose to be attached to. Maybe Nazrin's over-attachment to Bishamoten and her actions reflecting solely on that singular fact is to Shou the reflection of what she just did, and is kicking her ass for it. Seems like hypocrisy, in a sense, but it's still defendable. Kind of sort of.

1. She's trying to teach Naz a lesson. You don't do tat with smiles and sunshine.

2. She probably would like Naz's support and friendship but that requires trust and trust requires confronting truths.

Shou is evidently one of those "rip the band-aid off in one go" types.