Dungeon Warg (in Cirith Gurat) has Surge and the text "When Revealed: Dungeon Warg makes an immediate attack against the player who controls the most allies."

It so happened that between Thalin questing and a Gondorian Spearman with Spear of the Citadel that the Dungeon Warg (3 life) was killed when it attacked. Does the Surge still trigger?

Yes. Thalin only stops Surge because he deals his damage in the weird space between "taken from the top of the encounter deck" and "actually being revealed". When you are defending, this Warg has reached the "actually being revealed" stage and so Surge has been triggered.