I want to say that the glowing is a thing the other monks do (when actively using their powers?), but I'd have to check.
It's hard to imagine that baby Hrathen got a special neck-snapping bone within the first few months at Dakhor. I guess I imagine that there's a base set of bones that everyone gets before possibly being allowed to branch off into specialized bones?
As for Dilaf being old and Hrathen being more fit, well... Dilaf does have the body of a 20-something, and I assume that's not superficial (like, he can probably eat all the brunost he wants without gaining any weight). Plus, I don't think there's anything that implies that he's not fit as well, even if not to the same level as Hrathen. So if anything, I'd say Dilaf's age would be an advantage, given how much more experience he has with... having weird bone powers?
Honestly, the only thing I can think of is that Dilaf is just so short that he wasn't able to use his legs to try and free himself by pushing back against Hrathen or, you know, just kicking him in the face. Though, that would only explain why Dilaf wasn't able to fight back, not how the heck Hrathen was able to kill him single-bonededly while half-dead. And as much as I love joking about The Littlest Gragdet, even I don't think he's this short.
In conclusion, that whole thing was really weird and I'd call shenanigans on it, but the alternative was apparently Dilaf being cursed just like his wife was, which really isn't how The Good Guys should be defeating their enemies, so I'm forced to give it a pass.
Edit: ohhh, that's April 2021.