Well... that one's tricky. On one hand, there are blood mages, abominations, and demons roaming around the tower. On the other, the Templars are controlled by the Chantry. Besides, the Dalish don't blame mages for the fall of Arlathan and the Dales like Fenris does. They blame humans in general. 
You're right, that one probably could have gone either way, but remember, you're being allowed by humans to eliminate other humans and city elves, who had also gone abomination (meaning we had the entire "abominations get hunted down" Dalish rule to take into account). So when the choice comes down for a Dalish to "save humans" and "eliminate humans", well, that's an easy option.
Fenris would blame the mages for loosing his car keys. Arlathan fell because of the mages, Dales fell because of the Chantry, assign blame where it belongs, now try telling that to the people who have been oppressed for over a millenia .To all fairness I don't think a Dalish could hate mages in general, considering the fact that every clan is led by a keeper who has access to .. you said it.. ancient magic.Also It'd be tad hypocritical for a Dalish to have a mage as a leader but also hate all mages because of Arlathan.
Tevinter are still major slavers. And saving the Circle means assisting the Chantry, as does annulling the Circle. Either way you're going to be helping out the Chantry, but annulling the Circle denies them the resource of the Circle.
Am I the only one who played a diplomatic Dalish? 
Probably not, but when your character who believes that :
- Humans cost Elf kind their immortality
- Tevinter cost Elf kind Arlathan and are slavers
- The Chantry cost the Elves the Dales
It's very difficult to care when you're forced by blonde-human-who-won't-stop-talking to care about the human nation of Ferelden.