I am quite interested in finding out how this all happened. I doubt that many Templars would quickly sign up to become crazy monsters so they may have been forced or tricked into it or they might just be nieve. Cassandra's interrogation of Varric made it seem like the details of what happened in Kirkwall were not widely known. Maybe they did not know what they would become, or at least most of them didn't.
Also, are they all the Templars who no longer serve the Chantry?
From what I gather (granted I have not absorbed one bit of any other DA media other then the games and am relying on the Wiki) all Templars left the Chantry as well as the Seekers of Truth. And yeah, I think...it could be any number of reasons why initially SOME Templars are turning to Red Lyrium as a source of power. One t makes them more powerful and gives them magic like abillities which could be compelling. Two they could be compelled by the Red Lyrium to give it a try and is twisting their motivations. Some of them might not believe the horror stories, chalking it up to mage propaganda, or some of them might believe they can handle it and it should be no worse then regular Lyrium and what harm does that cause? After all I have been on the stuff for years and it hasn't harmed me! After some of the Templars started turning into Abominations though I imagine the further source of recruits is mostly through forced conversion. Red Templars capturing other Templars and Civvies to make more RTs.
Ultimatly it boils down to eseclation. For the first time in years the mages are rebelling, that is an escelation, and instead of calming the situation down the Ts are eseclating the situation more, so the RTs are just doing the next logical step and got caught in the cross fire of the RLs corruption.