Paul had chosen in his heart which way he would walk on. He wasn't struggling like that just because Rick ordered him to do so, but because he promised to give his freedom back if he could stop Dean.
"Paul, listen to me. You can't take your freedom back just by trying to kill me, because you can't do so. The first order that I ever gave to you was that you couldn't hurt me and needed to protect me when I am under life threat, after all. However, I promise you that it won't last forever. If I die, you will be a free man. And I am willing to die to do something very important right now. So I need you to stop Dean until I can accomplish it."
Paul knew that Rick wasn't lying at all. He heard from the other members of his group about his upcoming death.
