
Originally Posted by
-Dalliah-
In the introduction panel for World of Warcraft: Wrath of the Lich King, and the following afternoon during the WoW Lore Panel, Chris Metzen elaborated on the status of Malygos and the blue dragonflight. It was revealed that Malygos has begun to regain much of his sanity. Tyrygosa - having taken the Nether Dragons under her wing - has brought the ethereal dragons to Northrend. The exposure to their physiology has helped cure Malygos and the Blue Aspect's sanity is returning to him.
Malygos, with clearer eyes, takes note of the large number of mortal spellcasters running around Azeroth, recklessly using arcane magic, and worries they may bring the Burning Legion to Azeroth in force again, as the Highborne once did (one of his last few memories before going mad). Thus he deems the lesser races' use of magic as unacceptable, and he proceeds to declare war on all magic users, particularly the Kirin Tor of Dalaran. Dalaran has therefore moved to Northrend, as a capital for both the war against the Lich King, and the Kirin Tor's fight with Malygos. The red dragonflight - keeping Alexstrasza's promise to Rhonin after the events of Day of the Dragon - has stepped up to defend the mortals from their cousins' crusade against spellcasters.
TL;DR Malygos was angry because he didn't want the reckless usage of arcane magic to draw the burning legion back to azeroth, and he wasn't mad at all (didn't know that until now).
I really feel that his storyline in wow could've been so much better...