He has his reasons. Fenris comes from the country where mages are free, and where he was enslaved by them. Anders was oppressed by templars and hates them, but I don't see you judging him, although their reasoning are the same. If the situations were reversed and the game took place in Tevinter, where mages are oppressive class, would you still make this claim?
Besides, Anders and Merrill are Hawke's friends, not his.
Becoming a Templar is a choice (and the job description is not exactly veiled in secrecy), being born a Mage is not. Anders hates Templars because of their choices. Fenris hates every Mage Everywhere because one population of free Mages in one country misuse their power. If his anger where concentrated on Blood Mages then I would be more sympathetic but he desires EVERY Mage to be treated no better than he was just for being born as they are.
^ He has a respect and liking for Bethany, but since she goes away at the end of Act 1 there really wasn't time for her to become a "friend." So really, the only mage friend that Fenris has is Hawke.
That was entirly his choice. He was openly hostile to both Anders and Merrill from the begining. If he had set aside his prejudices he could have found some common ground in that he and Anders both dislike Blood Magic and a common history as an Elf with Merrill.
This is why I said the lack of empathy he gets make me sad. All people see is omg anti mages without considering why might he feel that way (and there are some really gross comments about genocide and ethic cleansing thrown around too). They basically expect that he should be completely sympathetic with the people who abused him, when he has no reason too. Of all the mages you meet, there's only a couple that don't actually serve as a counter for his experiences.
Don't get me wrong, I like Fenris and his treatment at the hands of the Magisters was inexcusable. I understand his feelings towards Tevinter and even Blood Mages in general but his hatred of Mages everywhere who have done nothing to him is unwarranted and little different than the racism he himself experiences as an Elf or the mistreatment of Castless Dwarfs simply for being born in the wrong family. That said, I do see how is experiences have led him to see the world as he does. I don't see why he would then befriend an Apostate Mage anyway.
There are a good number of Mages we meet, and play, that are contrary to Fenris' views. Wynne and all of the Mages of the Circle and the Mages of the Grey Wardens who fought against the Blight. Possibly The Hero of Fereldan, who, if he/she didn't give their life to stop the Archdemon went on to stop The Mother and possibly other heroic deeds. Bethany, who if not killed defending her mother joins the Wardens, Hawke's father who made a life for his family without enslaving others, Hawke's many possibly heroic deeds, including helping Fenris and Anders who helped stop The Mother and risks his life and freedom healing perfect strangers in Dark Town.
I would argue than many of the Blood Mages we meet throughout the games were pushed down that road by the Templars. It was not their goal to enslave others but to survive. I see them as no different than Fenris who freely admitted that he too slaughtered his way to freedom.
P.S. I was serious about my first question. Why is Fenris the only DAII character with a theme song?