The light will stand with whoever has a strong belief their cause is righteous and has not if any flick of a doubt, which is why you see the Scarlet Crusade using it even if they are purging Lordaeron of any life form their deem corrupted or even Yrel with her holy Crusade in the alternate Draenor.
Liadrin and the blood knights can simply have sided with Sylvanas and in their mind believe is the right choice as for them this will guarantee the safety of the blood elf population and Azeroth overall.
Magatha siding with the shamans and becoming a follower pisses me way more than the paladin debacle