Please tell me that this whole Lightbound thing is just the new scarlet crusade, and that "High Exarch Yrel" is just the latest nathrezim to take over a religious people group in disguise. It would be way more believable than Yrel acting this far out of character.
So, they convince the Mag'har to join us and only then mention that the new Warchief is not quite a tyrant. That's hilarious.
This comes off a surprise to me, but maybe the reason Yrel went nuts was because of Xe'ra. Let's not forget the Naa'ru aren't all one mind. Its very likely, like many other groups, the indivual naa'ru have their own thoughts and emotions. (Look at Sargeras and the other titans). Its very likely that Xe'ra was an extremist, and was able to slowly overtime turn Yrel over to her thoughts. The dangers of "unquestioned faith" is evident when you take into fact people can make stupid choices when they get between their concience and their faith (and the possible misinterprations of such)
The great Alliance saviour of Draenor, reduced to a fanatical cult leader, a Draenic Scarlet Crusader...Honestly? I love it! :D Yrel will serve as a reminder that absolute devotion to one side will corrupt absolutely. She didn't have an Illidan to remind her of that.