Their reasoning is that the Eredar originally was a race of philosophers and magi. After Sargeras' transformation, he offered them power under his command. Turning them into demons apparently. Only a small amount left to be good people lead by the Light. Though they kinda make a snafu. Eredar are originally demons while Sargeras is still a sane Titan. Original version says Eredar drove Sargeras insane. Now they say Sargeras drove the Eredar evil. It's pretty damn hard to understand why they didn't decide to say the Nathrezim didn't do it, or something. =/