I don't know if it appears that way in the book or not, I don't recall, but if the producers did it intentionally they may have had a symbolic intent.
In any case, there is no intrinsic evil to the gesture, albeit the traditional form is with the right hand. I have blessed myself with my left hand when my right hand was occupied (holding or carrying something), and it is perfectly fine.
- If your right arm was amputated what would you do? <You would use the left.>
So, the movie may have some veiled symbolism, since the left is sometimes associated with something sinister, as noted in the root of the word for left in Italian, sinistro.
- Were they intended as a demonic force?
Maybe, but we can only speculate. It's only a movie. I personally wouldn't give it much concern.