Well, on RP ground it will happen and cannot be prevented, no matter if the character actually possesses the power to do so game play wise or not. There will be 'evil' Templar, like every community got their crop of 'evil' / 'good' people.
A Templar cabal who uses dark magic to destroy evil I can see definitely working. It is controversial certainly, but the cabal might think along the line 'it gets the job effectively done'. This would be questioned by many, Templar or not, but it is how quite a few people think and it actually enriches the environment in my view.
For your take, I'd then concentrate on cabal level really, to only join a cabal who doesn't allow any forms of dwelling into 'darker arts'.