Aluneth is a snarky punk and his comments should not necessarily be taken at face value. Having been described as unruly, capricious, and unwieldy.
I think the primary source of his bias regarding elves and humanity is how they treated him. Meitre, his Night Elf user, merely drew upon his powers without binding him. This seems evident especially in her comments about Aegwynn.
She lives? No! I will never let her bind me again! (in the Cathedral of Eternal Night, when the Image of Aegwynn appears)
Ah… merely a lingering echo of her power. Nothing to be concerned about, child. (Cathedral of Eternal Night, to Aegwynn)
Even her echo prattles off endlessly! Do you see now what I had to deal with? (in the Tomb of Sargeras, when Aegwynn’s echo appears)
Do not trust her ramblings. I am the only true power here. (in the Tomb, after placing all the Pillars of Creation)
You are better off without her. (going into the lower level of the Tomb)