I wouldnt describe him as a con artist at all.
He conned Rey into thinking that he was with the Rebellion,he conned the Rebellion into thinking that his job as a Stormtrooper was something more important/significant then what it was,and he conned the Rebellion into thinking that his reasons for going to the Starkiller base was to both destroy the Starkiller weapon and to rescue Rey, when he actually his real and sole reason for leading them inside was to rescue Rey.