As to your original question, I usually do multiple playthroughs. First, in DA2, I did think it would be hard to side with the templars, because of the extremism of the templars in Kirkwall (it's worth remembering that most Circles are much better than the one in Kirkwall, by everyone's admission). However, I can see how the extremism is a product of the fear of the mages, much as many mages turned to blood magic in fear of the Templars in Kirkwall. The fear runs both ways, and it's a complicated issue.
Yes, mages are people. No, they shouldn't be abused, nor should they be made Tranquil if there is another way. But how many can they be allowed to kill? Not only do they possess great, unstoppable power themselves to kill if they so desire but they can be possessed and become abominations that kill hundreds, thousands, etc. After playing through DA2 as a mage-sympathetic, Anders-romancing rogue, I could see how I could side with the Templars. Mages were trouble all through the game. Yes, you could say some turned to it because so many of the Templars in Kirkwall were awful (but some were not) and it was overly oppressive, but nonetheless, they caused a lot of trouble, evoked a lot of blood magic, and so forth. Then, there is Tevinter. Fenris is a victim of the mages in Tevinter, and magic could just as easily be the oppression as the Templars could. We see many historical examples of that in Thedas. Then, there is Hawke's mother; that mage was not in the Circle, not using blood magic because of Templars, but rather for his own personal gain. Mages are dangerous. Something has to be done.
When I sided with the Templars in Kirkwall on another playthrough, I still was not evil and didn't wish to kill the mages truly. It started small. I wished the mages controlled. It was only when everything was spinning out of control that Meredith called for the rite. People were going to die either way. By that time, I saw her side more. I'd spoken with her (you only get to really understand Meredith on a pro-Templar playthrough, as there is additional content there). I thought it was the better of two evils, lest the whole city be brought down by abominations ("I thought" = my roleplay there).
In truth, my inclinations are more towards siding with the Mages (though not to disband the Circles) but that doesn't mean I can't see both sides of the issue.
The templars are specially trained to fight such demons (and mages).
The templars are specially trained to track and resist mages, not demons. They have some knowledge and training of demons, but the purpose of a templar is to prevent demon possession and track down apostates, not to fight demons outright. It is a rare individual who faces a demon and lives and is not something Templars can ever really be prepared for.