I don't see it as making him two-dimensional. It seems to be a natural progression of his character. Because of the actions of the Ancient One and Kaecilius, and to a lesser the attitude of Strange, he's come to view sorcerers as a threat to the Earth. Given that it almost got wiped out by Dormammu, he's not totally unjustified in that. So after thinking about it for a few months he decided the best way to protect the Earth would be to take out the other sorcerers. Now maybe the end up going a different direction in the sequel, which would be disappointing, but if they stick to what we see at the end of this film there shouldn't be a problem.