"We can take the naaru's core heart to purify our magic pool" is a pretty big leap from "one can siphon light magic from the naaru". And would require killing the naaru first and experimenting on their hearts, whereas Velen had direct divine information about M'uru's intentions.

So... If I drink priestess blood I'll become holy?

So Kael KNEW about Q'T joining the Horde all along! What new aspects do you think it brings to his character?

The Sun King did everything for his people. Indeed, Lord Kil'jaeden was going to deliver us, but for the intervention of the treacherous Shattered Sun scums and Liadrin the Naaru-slave, in league with those blue-skinned mongrels. The anti-Sunstrider propaganda of Lor'themar's is sickening; I miss the days when Quel'danas was under the wise leadership of Lord Brutallus, who was still but an insect compared to the wisdom of the Great Lord Kil'jaeden (or so the loremasters told me; for unfortunately that miscegenous couple, Liadrin and Velen, took away our chance to experience His enlightened leadership). May the memory of Kael'thas Sunstrider live for ever; he was the true heir of Azshara.